You, the contestant, is holding onto the door-handle, and behind it there might be a goat or a car ... you don't know yet.
And the 2nd door, which has been opened by the host, reveals a goat.
And then you have to decide whether you will open the door you are holding on to, or you can switch to the other, still closed door.
One of the two doors that are still shut has a car behind it, and the other has a goat behind it.
So your choice is 50-50 ... it seems that it doesn't matter which you open.
Common sense tells you that.
But common sense is wrong !!!
And here's why ...
The bottom row tells you two things ..
If you stick to the door you chose, you'll only win the car 1 time out of 3.
But if you go to the other door, you will get the car 2 times out of 3 !!!!!
Well worth the short walk to the other door !.
But ... but ..... you can see that there's a clash between the maths and the brain.
So ...here's a helpful video all about it ....