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. Ease of starting is at an all-time high: Platforms like Amazon, Airbnb and Fiverr offer instant access to paying customers. Kindergarten teacher Becky Powell's side hustle — making worksheets for other educators — brings in six figures per year. 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.

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