Evergreen content, which is described as that SEO content which stays relevant for a long time after its initial publication, is a good way to generate income. Comprehensive research statistics and case studies, such as social media marketing trends for the last five years, and detailed how-to guides, such as a beginners guide to using Twitter for business, are always going to be sought after people who wouldn't mind paying small amounts for access to the information.
What’s also really important to realize here is that when I took the exam I was teaching people to study for, I didn’t get a perfect score. In fact, I didn’t even get close to a perfect score. I passed. But I also knew a lot about this exam—way more than somebody who was just getting started diving into studying for it. And it was because of that, because I was just a few steps ahead of them, that they trusted me to help them with that information. To support this, I provided a lot of great free value to help them along the way. I engaged in conversations and interacted in comments sections and on forums. Most of all, I just really cared about those people, because I struggled big-time with that exam myself.
For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.