Supplemental Income #18 is to make your own version of an electronic book or ebook for short. If you are an expert in a certain field and if you are uncomfortable with supplemental income #17 which is to become a teacher then you might just do good as an ebook writer and sell it for a decent supplemental income. The idea on this one is pretty much the same with #17 because both refer to the idea of imparting knowledge as a good source of supplemental income. The only difference with #18 is that its for people who are more of the shy kind of person or otherwise referred to as introverts since you can still impart knowledge and earn an income off of it without the need of interacting with people whereas becoming a teacher involved a more personal contact.
Supplemental Income #19 is to know how to effectively optimize websites to make it more search engine friendly. This one is hot right now because a lot of people want to have an online presence to have good traffic that will become potential customers and clients and search engines are one of the most effective and least expensive ways of achieving just that. So if you are good at search engine optimization or SEO then you can have a good solid supplemental income from that alone. Although search engine optimization is constantly changing due to search engines changing their formulas and algorithms from time to time to prevent being cheated and tricked by hackers who try to make money from it, if you know the basics of how to set this up you can pretty much have no problem catching up on the latest strategies at all. This may require you some hard work that is why its better to choose this kind of supplemental income if you really have a passion for this kind of stuff.