A timer and a simple rule: stop it at exactly four seconds.
Got this game idea while I found myself routinely trying to stop a timer at a 'round' value while at the gym. Also heard some stories about how people used to play this sort of stopwatch games on digital watches, back when Casios were the thing.
Squared is a new puzzle game for iPhone. It will change your life... or it won't. But you'll still definitely enjoy it. Challenging enough to keep you interested but simple enough to feel like you're never stuck.