The Monty Hall Problem

            Imagine that you are on a game show. The premise is simple; there are 3 doors with 2 of them having a goat behind them and 1 of them having a million dollars. You are allowed to pick 1 of the 3 doors and you get to keep whatever is behind the door. After you pick a door, however, the host opens one of the doors that has a goat in it and asks if you want to switch to the third door. This is where this problem gets tricky. You may think in this situation that switching or not switching is pointless because it is a 50/50 chance anyway, but this is not the case. In fact, switching doors will allow you to win two-thirds of the time, and sticking to your initial door will only let you win one-third of the time. 

This statistical fact seems odd but it begins to make more sense when you start to think about it. When you pick the initial door, you have a one in three chance of picking the door with money behind it and a two in three chance of picking a door with a goat behind it. This means, statistically speaking, you initially will choose a door with a goat. Now when the host removes the other door with a goat behind it, switching will give you the money. This means that, as long as you switch, you will win if the first door you pick is a goat, which is two-thirds of the time. 

Source: 

https://betterexplained.com/articles/understanding-the-monty-hall-problem/