So you think you understand gambling odds, eh? Well, try this...
Imagine you're a contestant on a game show, and you are given the choice of three doors and asked to pick one. Whatever prize is behind the door you choose, you win. You are told that behind one door is a car, but behind each of the other two doors, a goat.
You pick a door, say No. 1, and, before opening the door you have selected, the host opens one of the other two doors, say No. 3, to reveal a goat. He says to you, "Do you want to stick with the door you've chosen, or do you want to change your mind and go for door No. 2 instead?"