Makey Makey is a little circuit board that comes with a set of alligator clips. You can attach them to anything even mildly conductive (a body part, a glass of water, alphabet noodles, paper clips, Play Dough, or fruit for example) and use that thing to control your computer as though you were hitting the keyboard or moving the mouse.
Turn a bunch of bananas into a piano. Turn your friends into a synthesizer. Turn a trampoline into a slideshow controller. Turn your hand into a game glove. The possibilities are endless.