Children! I've decided to give you some money for Christmas this year.
Owen: Ripper.
Charlotte: How much?
I'll leave that to you. But if you're too greedy, you will receive nothing.
Owen: How do you mean?
Well, I'll just write an amount of money on this piece of paper, and cover it up. Then you tell me how much you want. If you ask for the amount I've written down, or less, then I shall give you what you ask for. But if you ask for more, you will be punished for your greed, and will receive nothing.
Would you like to practise? I've written down the amount. How much do you want?
Charlotte: five dollars.
Owen: twenty dollars.
(Uncovering paper) As you can see, I wrote down thirty dollars, so you would each have received the amount you wanted. Now we'll do the real thing.
Charlotte: twenty dollars.
Owen: five dollars.
Well, as you can see, I wrote down ten dollars. So Owen will receive the amount he asked for, but greedy Charlotte will receive nothing.
For further information on game theory, see Von Neumann and Morgenstern's original work or a modern introduction such as that by G Owen.