Here's the effect I really like. It appears to be just the simple guessing game, but I think it's the risk that makes it interesting. If spectator manages to trick the performer (and his offered that challenge), he can win some money.

https://www.youtube.com/watch?v=zzkevNyWIck

What do you think of these kind of effects? Do you perform something, where you risk (at least in theory) loosing your money?

I would love to hear your opinions.