I need a new bum

I need a new bum! Mine's got a crack. I can see in the mirror a crack in the back. What to do when you need a new bum? Should you get one that's blue or yellow spotted? A Chevy bum, a rocket bum that's all fire and thrust, or a robo-bum? The options are endless - but wait, Dad's bum crack is showing too? Maybe this is contagious.