you have 2 ashtrays, there is a coin under one of the ashtrays. You have one ashtray under your hand and the other is not. The location of the ashtrays has no bearing on the odds. They are still 50:50. The current odds are not dependent on previous odds.
Wrong @flyboy - want to bet on it $) ?
If we started with 2 ashtrays the odds of the coin being under the ashtray under my hand are 50:50.
However we started with 3 ashtrays so the odds of the coin being under the other ashtray are 2:3, I have twice as much chance of winning if I change.
There is no trick here just maths, but it is very counter intuitive.