ExplainThatStuff! is a "FREE online science and technology book."

Originally Posted by ExplainThatStuff! website
Explain that Stuff is an online book written by British science writer Chris Woodford (author of DK's best-selling Cool Stuff books and Atoms Under the Floorboards: The Surprising Science Hidden in Your Home). It includes 450 easy-to-understand articles (plus around 2000 photos and 550 illustrations), covering how things work, cutting-edge science, cool gadgets, and computers. We take the "pain" from explain and the "tough" out of stuff! There's more information on this website than in your average expensive science book—and it's all completely free to use! Explain that Stuff also helps to support curriculum learning (conventional STEM education and home-schooling).
Written in a question-and-answer format, the articles pique interest and have broad appeal.