Watch this fascinating video and if you still don’t understand how the math works, see my comments below.
To further clarify, since the host will never open the door with the car behind it after you make your first choice, the crucial thing to consider is: what are the odds that the door the host didn’t open has the car behind it? If you initially pick the door with the car behind it then both remaining doors will have a sheep behind it. The odds of this happening are 1 in 3. If you initially pick a door with a sheep behind it, the host then must open the other door with the sheep behind it leaving the remaining door the one with the car behind it. The odds of this happening are 2 in 3. Thus, you have better odds of on your second choice choosing the door the host didn’t open rather than sticking with your initial pick.
Strange how that works, no?