Yep, I figured that was an easy one.
Let's say you've got a vending machine that sells either Coke or Pepsi. There are three buttons on the machine. Pressing one will get you a Coke, another will get you a Pepsi, and another will give you one of the two at random, just for fun. One day some joker rearranges the labels on the buttons so they're all wrong. You don't really remember which is which, you're too lazy to open up the machine and tinker with it, and you're thirsty anyway. How many drinks do you have to buy to figure out how to rearrange the labels?