One person creates a challenge that is impossible and the next person has to come up with a creative way of completing it. For example:
Challenge- Set an ice cube on fire
bob said:Challenge- Eat your own head
etc. so:Geoff said:I clone myself and eat his head. Since we're exactly the same, I'm eating my own head.
Challenge- Prove that there is an afterlife
Challenge- Set an ice cube on fire
Hope this takes off