Kneaded erasers are a great tool because they erase cleanly without leaving bits behind and they can be sculpted into just the right shape for whatever you need to erase. (Fun fact: they're also fun to play with!) To clean a kneaded eraser, just smoosh it together until the darkness is spread throughout the eraser. It's time to replace a kneaded eraser when it is no longer possible to knead either because it is completely hard or everything is pretty much black. You might be surprised at how long that takes.