Search for the Monty Hall problem.
You are at a show. The host shows you three doors and ask you to pick one. Behind one of them is a car and behind the two others are nothing. You choose a door. However, before opening the door you chose, the host (he knows what is behind each door) open one of the two doors you didn't picked and shows that there is nothing behind it. He then asks you if you want to change your mind. Would you stick with your original choice, or change to the other unopened door (since the host opened one) ? god i feel stupid today