found out yesterday that my american friend has one of these. His dad used to tickle him when he was young, to make him laugh, so i was told whenever you look him in the eye and rub your fingers together (think "money" gesture) he bursts into uncontrollable laughter.

I didn't believe it until it was demonstrated. It was unbelievable. He genuinely started gutting himself and had to go into a different room to calm down, then came back looking pretty annoyed. It was amazing.

I'd never seen anything like it. Anyone got any similar stories?