Caesium is a Metal which can combust spontaneously with air. It is used for atomic clocks.

Never call Caesius fat, or you’re in trouble. It is actually quite athletic: It can hit a traffic light with a tomato from half a mile, making it turn to red. Also it is obsessed with the clock: It asks everyone what time it is. Who can’t answer quickly enough, will get pelted with whatever is lying around.