It’s probably happened to you: a friend says, “I know this palace like the back of my hand,” and then proceeds to get you helplessly lost. Well, it could be that they really did know it like the back of their hand. Because researchers have found that people don’t actually understand their hands as well as they think they do. The study appears in the Proceedings of the National Academy of Sciences. We all have “mental representations” of our body parts. So when our hands or feet are out of sight, we still know pretty much where they are and what they look like. But just how accurate is our metal picture?