The bit that gets me... If he opens one door, which is a goat, so there is 1 goat and 1 prize remaining... then the chance should be 50% staying put or switchin

The bit that gets me... If he opens one door, which is a goat, so there is 1 goat and 1 prize remaining... then the chance should be 50% staying put or switching, right? That's the part that gets me. The whole bit about 3 doors and opening one of them and "do you want to switch" is all a ruse, it seems to me. At the end of the day there are two doors, one with a goat and one with a prize, and however you choose between 2 doors, you will win or lose 50% of the time. Right? Aargh the SAN loss...