A couple of months ago I needed a quick weekend project just for something to do, so I tried making a pair of puzzle boxes. The trick to open them is centrifugal force - there are pins that can slide in holes that are drilled into the lid, and no matter which was you tip the box at least two of the pins will be engaged and prevent the lid from opening. But if you spin the box around its central axis, centrifugal force will push the pins into the body of the box and allow the lid to be removed.
