Favorite practical jokes?

My personal favorite prank is to wrap a rubber band around the nozzle of the sprayer hose on the kitchen sink. Nothing like watching your roommate trying to get a glass of water and hosing himself down instead! But I bet you guys have even better ones. What're your favorites?

Ye olde hole in the styrofoam coffee cup.

Throwing a lit book of matches under thebathroom stall of a person taking a crap, watching them stamp it out its hilarius.