a bomb you throw and it explodes when you hit/get near it. a banana/walking enemy that walks back and forth across the track. a red tire/white wall tire that adds an extra life in 3SB.
Some people are like slinkies... not really good for anything, but you still can't help smiling when you shove them down the stairs.
Lasts about 30 seconds unless you collect another present, but... is not always active..when a weapon is fired at you either from front or behind you hear a warning beep and have a certain (very short) amount of time to hit the fire button to activate weapon and hit the plunger/ muffin whatever which is heading towards you. It can also be used to clear bananas or karts but has to be timed correctly (maybe baseball bat better, in that case)