Show two matchboxes to the audience, one open and one closed. Try to push the drawer of one box closed, and the other box drawer will open! You can adjust the matchboxes into many positions, but still, both drawers won’t close at the same time. Next you completely separate the matchboxes, and the drawers STILL WILL NOT CLOSE! Just when you’re about to give up, the drawers both close together!
Easy to do, and completely amazing – this effect always gets a big response!