You are given $100 and asked to share it with someone else. You can offer that person any amount and if he accepts the offer, you each get to keep your share. If he rejects your offer, you both walk away empty-handed. How much would you offer?
If you were going to be the person the money was offered to, how much would you need to be offered to accept?
Interestingly, most westerners would apparently offer around half of the money and are likely to reject offers of less than around $40. Not entirely unexpected, living in a western culture that's definitely the way people appear to behave (personally I was going to offer most of it - not sure what that says about me).
However, that brings me onto this interesting article on cultural divides in psychology:
Again it's not exactly surprising. You would expect culture to have an influence on our decisions, especially when it comes to something as fundamental to western society as money. Somehow we always seem to forget this and expect the whole world to behave as we do.