Here’s a quick riddle:
You owe your friend two Dollars. You have a $5 bill, but your friend has only a $1 coin for change.
What would be a fair way to settle your debt that would not require you to get more change?
Answer: You toss the coin. If you win, you don’t give your friend anything. If your friend wins, you give him the $5 and he gives you back $1, so he gets $4. His expectation is $4 * 1/2 = $2, which is exactly fair.
Thanks for all the people who posted their answers in the comments!