I love to see curiosity in my kids. Whether they're trying to figure out the secrets of a magic trick or predicting where a marble will roll, I love to see their minds at work contemplating design and mechanics of how things work.
The Engineer Guy has some great, well produced videos that explain how many different things work. I like how the Engineer Guy explains things clearly and the visual demonstrations help so even I can understand what he's talking about. Whether you want to supplement a science lesson or you're just curious, these short videos are great for explaining how things work.