Probability - How does it work?!

This is also known as the Monty Hall problem. Easiest way to figure it out is to imagine there are 1000 cards. Imagine you picked one (1/1000 chance of getting the right one) and then 998 of the others were turned over. The person won't turn over the X card as he knows were it is, so there is now a 1/1000 chance your card is correct, and a 999/1000 chance that the other remaining card is; hence you should swap.

Great way of explaining this one, simplest I have seen. Nice.