Sarah and Jamie McCauley are landlords, YouTubers, Walmart pallet flippers, eBay resellers and Amazon product reviewers — and those are just their active streams of income.

The McCauleys make their money by researching what makes side hustles profitable, testing them and teaching others how to do the same on YouTube. The Grand Rapids, Michigan-based couple earned nearly $140,000 from eight streams of income last year, according to documents reviewed by CNBC Make It.

They’re particularly good at two types of gigs, they say: anything involving real estate and their YouTube channel itself, where they share their side hustle exploits with at least 146,000 subscribers.

“If you’re looking to just make some extra money on the side, maybe pay off a credit card debt or pay for a vacation, I think that is doable for nearly everybody,” says Jamie.

DON’T MISS: The ultimate guide to earning passive income online

The McCauleys are part of a side hustle revolution, a growing number of Americans who supplement income with multiple jobs. More U.S adults — about 39%, according to Bankrate — have side hustles today than ever before, whether out of necessity, precaution or a desire to increase their earning power.

Ease of starting is at an all-time high: Platforms like Amazon, Airbnb and Fiverr offer instant access to paying customers. But with competition also rising, it’s hard to build a side hustle that regularly brings in revenue.

Make It spoke with a selection of Americans with successful side hustles to learn how they built their businesses, and used them to fund a wide variety of financial goals. Every respondent highlighted four common traits that helped drive their success:

No matter what you sell, you need people willing to buy it. Jenny Woo says her side hustle is successful for a simple reason: She researches her audiences intensely, and tailors her products specifically to them.

Woo is an adjunct lecturer at the University of California, Irvine, a freelance business consultant and the teacher of an online course about emotional intelligence. Her one-woman side hustle, called Mind Brain Emotion, sells 12 different emotional intelligence-themed card games.

It brought in $1.71 million on Amazon last year, according to documents reviewed by Make It.

Woo’s first deck of cards, “52 Essential Conversations,” was tailored toward parents who — like her — wanted to connect with their kids and build their emotional intelligence skills. She joined parenting Facebook groups and observed users’ posting, commenting and liking habits, she says.

After selling $10,000 worth of the game in a 2018 Kickstarter campaign, Woo kept researching. She conducted a survey of her consumers, and learned that “overwhelmed” teachers looking to support children’s social and emotional development made up a significant portion of her audience, she says.

Her second deck, “52 Essential Relationship Skills,” was built for those teachers. It didn’t sell as well as her first deck, but it taught Woo that she could broaden, and combine, her audiences.

Woo applied that lesson to her third game, “52 Coping Skills.” She started with her own experiences working with college students during the Covid-19 pandemic and combined it with her continued research on teachers and parents, she says.

It’s now Mind Brain Emotion’s top-selling game, says Woo.

Woo sells on Amazon, which has a broad reach, to collectively rope in Mind Brain Emotion’s hyper-specific audiences. Tim Riegel’s products have a more singular customer base, so he sells on Etsy, a marketplace known largely for homemade and handmade goods.

Riegel, a full-time general manager at a sheltered workshop, makes firepits from recycled tank ends in Lamar, Missouri, and sells them under the name Mozark Fire Pits. His average product weighs 225 pounds, and sells for $950.

Mozark Fire Pits brought in approximately $202,000 on Etsy last year, according to documents reviewed by Make It. Riegel maintains a 40% profit margin, he says.

Riegel chose Etsy over platforms like Amazon, Wayfair and Overstock because it felt more user-friendly, and a better fit for his personalized products, he says. He also sells on Facebook Marketplace, which costs him more in advertising — but less in shipping costs for customers within a 200-mile radius, he adds.

That kind of platform analysis is valuable, no matter what kind of side hustle you run.

If you sell a service, instead of a good, you might consider platforms like Fiverr and Upwork — popular among photo editors, marketing writers and voiceover artists — or Taskrabbit, known for labor-intensive side hustles like cleaning or repair work.

Or, opt out of those platforms entirely. If your gig is something that many other people also do, try finding marketplaces with more narrow niches like Contently, Skyword or ServiceScape, recommends side hustle expert Kathy Kristof.

“One of the problems I see with a lot of freelancers is that they go to the best-known online platforms … and those platforms are so saturated with people who have been there for, often, decades,” says Kristof, whose blog SideHusl has reviewed more than 500 different side gigs.

No matter your platform, you’ll need to stand out. A good listing can help: clear and concise, written for your intended audience, free of typos, with high-quality graphics and some search engine optimization (SEO).

Becky Powell, a kindergarten teacher based in Beaverton, Oregon, has a side hustle selling worksheets for other educators on an online platform called Teachers Pay Teachers. Many of her worksheets focus on her personal specialty, teaching children sight-reading skills.

Her side hustle didn’t take off until she embraced SEO. When she uploaded her first worksheets, she titled them, “Creating sight words with pattern blocks.” Sales slowly trickled in.

Her husband Jerome, who has a business background, suggested a simpler title, like “Hands-on sight words.” The sight-reading worksheets quickly became her bestselling products, Powell says.

Powell’s store brought in $125,500 in 2022 revenue, according to documents reviewed by Make It. Her husband also sells worksheets on the platform, and they’ve used their combined earnings to fund vacations and pay down their mortgage and student loans, Powell says.

“You have to have passion and knowledge,” she says. “You also have to have a business sense [and understand] SEO.”

Once you gain enough customers, work to turn your sales into positive reviews, so you appear higher in platforms’ search results, Kristof advises. Customer service, prompt shipping and quality control can usually earn you a good online reputation.

The McCauleys have a rule for their ever-changing collection of side hustles: “You either have to be one of the first to get there, or your approach has to be very unique and different to be successful,” Sarah says.

But being first or unique doesn’t guarantee long-term success. In 2020, the couple was early to a side hustle trend: pallet flipping. At local warehouses, they’d buy pallets of returned goods from Amazon, Walmart or Target. They’d unbox the pallets, discover their contents and resell the items for a hopeful profit.

From December 2020 to December 2022, the McCauleys made about $19,500 in pallet-flipping profits, they estimate. Their most popular unboxing YouTube video got 5.4 million viewers, translating to an additional $30,000 in advertising revenue, says Jamie.

Last year, more Americans hopped on the pallet-flipping trend. Pallet prices rose, resale values dropped and a slew of unboxing videos diluted the McCauleys’ viewership. “The pallets became not really worth our time … from the standpoint of time over money,” says Sarah.

Four years ago, the McCauleys would’ve simply moved onto their next side hustle. Now, they’re feeling the strain of constantly building new gigs from scratch, and starting to reorganize their income streams into a smaller number of longer-term projects.

Instead of flipping their current home renovation project in Northern Michigan for a profit, for example — something they’ve done multiple times — they’ll keep it as their own vacation house and part-time Airbnb rental, they say.

“We always knew [side hustling] was going to have an expiration date,” says Jamie. “It’s a young person’s game, to always be looking for what’s next.”

Want to make extra money outside of your day job? Sign up for CNBC’s new online course How to Earn Passive Income Online to learn about common passive income streams, tips to get started and real-life success stories. CNBC Make It readers can use special discount code CNBC40 to get 40% off through August 15, 2024.

Plus, sign up for CNBC Make It’s newsletter to get tips and tricks for success at work, with money and in life.

Almost 60% of the caloric intake of the average American diet is coming from ultra-processed foods, according to a 2017 paper published in the journal Population Health Metrics — and that’s concerning to health experts.

“Ultra-processed foods contain ingredients that we generally wouldn’t find in our kitchen, and they often contain high amounts of sugar and salt,” says Jinan Banna, registered dietitian and professor of nutrition at the University of Hawaii.

“They may also contain additives, and they often are stripped of their nutritional values. So they may have very little vitamins and minerals [and] fiber.”

Eating ultra-processed foods often can lead to a higher risk of developing health conditions like dementia, type 2 diabetes and heart disease, according to doctors in the American Medical Association.

That’s why Banna limits her consumption of ultra-processed foods and encourages you to do the same. Here’s the highly processed food that she never consumes.

“Some [ultra-processed foods] I would never consume, such as soda,” Banna tells CNBC Make It.

“Soda doesn’t have any nutritional value other than just calories in the form of sugar. So they’re empty calories, which don’t give us any of the nutrients that we need.”

When you drink soda, it is digested very quickly and can cause you to feel hungry, she adds, which can lead you to eat more food than you planned to.

Instead of soda, Banna opts for different types of tea and water, still or sparkling.

“Sometimes I drink a cold hibiscus tea. Plain water, of course, is a great alternative,” she says. “Even coffee can be an option, of course, consumed in moderation.”

A recent preliminary study that was presented last week at a meeting held annually for the American Society for Nutrition took a close look at dietary data collected in 1995 from more than 500,000 Americans between the ages of 50 and 71. The data was used to determine if there were connections between dietary choices and mortality rates over the span of almost 30 years.

Out of 124 foods, ultra-processed drinks was the No. 1 food that people who had the highest intake of ultra-processed foods consumed.

“Diet soft drinks were the key contributor to ultra-processed food consumption. The second one was sugary soft drinks,” the study’s lead author Erikka Loftfield told CNN.

Beverages make up a significant portion of dietary intake. So, these types of drinks — like diet sodas and energy drinks — are the processed food that people seem to consume more than others, Loftfield said.

The study also found that the lifespans of those who eat a diet high in ultra-processed foods may be shortened by over 10%, according to CNN.

As a rule of thumb, Banna recommends using the 5/20 method when checking the nutrition labels on the foods you eat.

“You can use the daily value,” she says. “That’s an easy way to know whether the food is generally high or low in a particular nutrient.”

Check the daily value percentages of specific nutrients like sodium, sugars or saturated fat, Banna suggests.

“The idea is, if [it’s] 5% or less, then the food is generally low in that particular nutrient. If 20% or more, you can consider the food high [in the nutrient],” she says.

“So that’s just a quick way to glance at the label and know if a food contains a little bit or a lot.”

Want to be a successful, confident communicator? Take CNBC’s new online course Become an Effective Communicator: Master Public Speaking. We’ll teach you how to speak clearly and confidently, calm your nerves, what to say and not say, and body language techniques to make a great first impression.

Plus, sign up for CNBC Make It’s newsletter to get tips and tricks for success at work, with money and in life.

Expats in Denmark are among the happiest with their work lives, according to the latest Expat Insider survey from InterNations, the online global community for people who’ve moved abroad.

The Nordic country ranks No. 1 for where people are most satisfied with their jobs, career opportunities and work-life balance, among other aspects.

That’s based on survey responses from more than 12,500 people living in foreign countries and reflects their feelings on four broad categories covering work topics, including their career prospects, salary and job security, work culture and satisfaction, and work and leisure.

Denmark ranks highest for two groupings: work culture and satisfaction, as well as work and leisure.

Some 84% of expats in Denmark are satisfied with their work-life balance, compared to a global average of 60%; a similar share are happy with their work hours and, on average, work 39.2 hours per week, compared with 42.5 hours for the global average.

Here are the top 10 countries where expats are happiest with their work lives abroad:

1. Denmark
2. Saudi Arabia
3. Belgium
4. Netherlands
5. Luxembourg
6. United Arab Emirates
7. Australia
8. Mexico
9. Indonesia
10. Austria

Ilana Buhl, a CNBC Make It contributor and American teacher who moved to Denmark, says moving from Texas to Copenhagen brought about lots of positive changes to her work life: shorter workweeks, stronger boundaries around off-hours, five weeks of paid vacation, and a better salary to cover a lower cost of living.

Outside of work, she also enjoys robust public transit, public health care, affordable day care, and ample maternity leave paid for by her employer and the government.

Many of these factors contribute to the fact that Denmark consistently ranks as one of the happiest countries in the world, according to the annual World Happiness Report.

Coming in at No. 2 is Saudi Arabia, where a majority, 75%, say moving there has improved their career prospects, versus 56% globally. The majority, 63%, of newcomers relocate to Saudi Arabia for work, where they say they’re satisfied with the state of the local economy.

Meanwhile, just 35% of expats around the world say they moved to a foreign country for job-related reasons.

The biggest downside to working in Saudi Arabia may be the long hours — expats working there report logging 47.8 hours a week for a full-time job.

Rounding out the top three is Belgium, where expats are highly satisfied with their job security, the local job market and their own career opportunities.

Expats in Belgium say they enjoy flexibility in the workplace, especially among the 68% who say they can work remotely, and their shorter-than-average full-time workweek of 40.8 hours.

Panama was recognized as the No. 1 best country for expats overall, according to the InterNations survey. The overall ranking accounts for broader life and financial factors, where foreigners say they’re satisfied with the quality of life, ease of settling in, working abroad, personal finance and an “expat essentials” index, which covers housing, administration, language and digital life.

Want to land your dream job? Take CNBC’s online course How to Ace Your Job Interview to learn what hiring managers really look for, body language techniques, what to say and not to say, and the best way to talk about pay.

Plus, sign up for CNBC Make It’s newsletter to get tips and tricks for success at work, with money and in life.

There’s no shortage of easy side hustles to try, for those who are looking to add another income stream. You can take care of people’s pets, rent out your house or apartment or do some virtual assisting.

Another side hustle worth exploring is market research. Before launching products to the public, companies often test run them with potential users to get feedback. They could be “trying out a new tech product and want to get somebody’s idea of how well it would work,” for example, says CEO and editor of Sidehusl.com Kathy Kristof. Often they’ll seek out people with specific backgrounds to make sure they test the right audience.

Here are several ways to do some market research and how much each could pay.

There are several ways to participate in market research: as an individual or in a group. If you’re interested in individual opportunities, consider signing up for the following websites:

• Respondent features various online surveys. You’ll create an account on the site and be asked about your background — family, health, country of origin, etc. Then the site will notify you when you’re a fit for opportunities. For example, a survey might ask for women, take 10 minutes and pay $25, or ask for web developers, take 30 minutes and pay $200 — though the specifics on demographic, amount of time and pay vary with each survey, according to the website. You’ll be paid via PayPal.

• User Interviews allows you to create a profile and either apply to studies or wait to hear from the site about which ones you’re a good candidate for. Instead of surveys, studies are often one-on-one interviews. The site lists examples like a 30-minute study paying $75 and a 45-minute study paying $200. User Interviews often pays with gift cards to companies like Amazon and Target.

• Rare Patient Voice is geared toward patients or caregivers, specifically. Illnesses and disorders that qualify vary but it can be anything from “asthma to diabetes to Alzheimer’s disease,” says Kristof. You’ll share your input in surveys, interviews or online communities and earn $120 per hour, according to the site.

Side hustle expert Daniella Flores, who runs the blog I Like to Dabble and who goes by they/them pronouns, has done surveys on Respondent themselves. “It was anywhere from like $15 to $100 per each of those,” they say.

You won’t be able to do it “for a set schedule,” they say, but it’s a good way to make some extra cash.

If you’re interested in participating in market research groups, consider signing up on sites like Find Focus Groups, where recent opportunities include a $180, 90-minute study for dog owners and a $150, two-hour Zoom call for homebuyers. Another option to consider is Fieldwork, where studies can pay $75 or more, according to Sidehusl.com.

Both sites offer many in-person opportunities and Fieldwork also includes some individual studies.

Because one-on-one interviews are more selective and focus groups are by default looking for a larger pool of people, opportunities for focus groups might be “little bit more common,” says Kristof.

Want to land your dream job? Take CNBC’s new online course How to Ace Your Job Interview to learn what hiring managers really look for, body language techniques, what to say and not to say, and the best way to talk about pay. Use discount code NEWGRAD to get 50% off from 5/1/24 to 6/30/24. 

Plus, sign up for CNBC Make It’s newsletter to get tips and tricks for success at work, with money and in life.

In some of the largest U.S. cities, earning six figures isn’t a benchmark — it’s become the norm.

The median household income is over $100,000 in six cities, four of which are in California, according to a new analysis by GoBankingRates. In San Francisco, at least half of households make $146,872 — the highest of all cities.

Here’s a look at the 10 wealthiest U.S. cities, based on 2022 U.S. Census household income data that’s been adjusted to 2024 dollars:

1. San Francisco: $146,872
2. San Jose, California: $146,143
3. Seattle: $124,715
4. Washington, D.C.: $109,300
5. San Diego: $106,007
6. Oakland, California: $101,421
7. Boston: $95,858
8. Virginia Beach, Virginia: $94,066
9. Austin, Texas: $93,004
10. Portland, Oregon: $92,274

The median income among the largest 50 U.S. cities as a whole is $77,046, which is more in line with what most Americans typically earn. In 2024, the median annual household income is estimated to be $78,171, according to data consulting firm Motio Research. 

But the amount that people earn can vary widely between cities. In Detroit — which is ranked 30th in population size — the median income is $40,574. That’s the lowest of all cities and more than $100,000 less than the median in San Francisco. 

The disparity largely reflects how the wealthiest Americans tend to be concentrated in a few of the largest U.S. cities. In San Francisco, there are 305,700 millionaires, which is second only to New York City’s 349,500 millionaires, according to wealth intelligence firm New World Wealth.

The wealthiest cities also tend to offer a high number of well-paying jobs in technology, finance, entertainment or health-care industries. Land scarcity is a factor, too, as it restricts new housing developments and drives up property prices, which can exclude lower-income buyers from the market.

Below are the median household incomes for the 25 most populous U.S. cities, ranked from highest to lowest. The average household income for the wealthiest 5% of earners in each city is also included.

• Median household income: $146,872
• Average household income for top 5% earners: $1,007,363

• Median household income: $146,143
• Average household income for top 5% earners: $775,803

• Median household income: $124,715
• Average household income for top 5% earners: $773,332

• Median household income: $109,300
• Average household income for top 5% earners: $772,837

• Median household income: $106,007
• Average household income for top 5% earners: $614,755

• Median household income: $101,421
• Average household income for top 5% earners: $691,594

• Median household income: $95,858
• Average household income for top 5% earners: $727,746

• Median household income: $94,066
• Average household income for top 5% earners: $502,395

• Median household income: $93,004
• Average household income for top 5% earners: $607,976

• Median household income: $92,274
• Average household income for top 5% earners: $550,487

• Median household income: $92,249
• Average household income for top 5% earners: $625,097

• Median household income: $84,913
• Average household income for top 5% earners: $442,133

• Median household income: $84,880
• Average household income for top 5% earners: $457,729

• Median household income: $84,836
• Average household income for top 5% earners: $439,505

• Median household income: $84,489
• Average household income for top 5% earners: $565,658

• Median household income: $83,440
• Average household income for top 5% earners: $820,758

• Median household income: $82,314
• Average household income for top 5% earners: $746,353

• Median household income: $82,019
• Average household income for top 5% earners: $552,377

• Median household income: $81,924
• Average household income for top 5% earners: $667,861

• Median household income: $79,588
• Average household income for top 5% earners: $644,063

• Median household income: $79,327
• Average household income for top 5% earners: $453,412

• Median household income: $79,262
• Average household income for top 5% earners: $408,068

• Median household income: $78,144
• Average household income for top 5% earners: $436,906

• Median household income: $77,463
• Average household income for top 5% earners: $486,335

• Median household income: $77,080
• Average household income for top 5% earners: $385,600

Want to stop worrying about money? Sign up for CNBC’s new online course Achieve Financial Wellness: Be Happier, Wealthier & More Financially Secure. We’ll teach you the psychology of money, how to manage your stress and create healthy habits, and simple ways to boost your savings, get out of debt and invest for the future. Start today and use code EARLYBIRD for an introductory discount of 30% off through September 2, 2024.

Plus, sign up for CNBC Make It’s newsletter to get tips and tricks for success at work, with money and in life.