Brief problem description:
You're on a game show. There are three doors. Behind two there are goats. Behind one there's a car. You're asked to open one door. Then the host tells you one of the doors in which there's a goat, and gives you the opportunity to switch doors.
Question: Do you switch the door?
Answer: Yes. You have greater odds if you do so.
Problem: But what if I picked the right door? The car is only in one of the doors, and what difference it makes if the host shows me one of the doors? What if I have the car?
Usual Solution: maths.
No complex maths, no fancy thinking. Here's the best explanation I've found on the web: