Some may have heard of this before, but it's a good one nonetheless!
You're playing a game - it's pretty simple, there are 3 doors, one with a prize behind and the others with nothing behind. You pick a door to open, however before you open it the game host arbitrarily picks another empty door (not the one with the prize behind) and opens it. This will therefore leave you with your original choice and one other door - one containing the prize, one containing nothing. He then gives you the option to choose either remaining door.
Question is, should you stick with your original door or switch to the other? Or doesn't it matter?
Save the whales, feed the hungry, free the mallocs.