A virtual whoopee cushion for your iPhone! This does pretty much what you'd expect it to do. Give it a tap and hear one of a half-dozen different fart sounds. Always amusing, no matter how old you are.
I would not advise you sit on it, no matter how tempting!
If you want to be sneaky, use the built-in 15 second delay timer. Simply press the timer, lock your phone without going back to the home screen and wait.