A pastime that involved placing one hand, palm outwards, on the top of your head and the other hand held in place between your teeth by the skin between the thumb and forefinger. In this pose you would run around the yard making a loud squeaking sound. When you approached a victim you would take the hand out of the mouth and hit the victim in the testicles then quickly replace the hand and run off squeaking. The victim had then been 'Yampy-Batted'. The only way to become a Yampy was to draw a bat - not unlike the one on Batman's bat-signal - on your arm with a pen. Anyone not so marked was a potential victim.

Another defence, assuming that you are allowed to defend yourself without being swarmed by Yampy Bats, would be to hit the hand on the head, causing them to bite through the skin between thumb and forefinger. This evolutionary flaw probably explains the lack of Yampy Bats in adult life.