Here's an old one to think over:

You're on a game show and you must choose one of three doors.

You win the prize behind the door of your choosing; behind 1 of the doors is a car, behind the other two are goats.

You choose a door, the game show host (knowing where the car and goats are) then helps you out by opening one of the doors and showing you a goat - he must always show you a goat, it's the rule of the show!

You now have 2 doors remaining, your chosen door and the last remaining door.

The host asks you if you would like to switch your chosen door.

Would you switch or not and why?