Are you familiar? Monty gives you a choice of three doors. Behind one is a new car. Behind the other two are hamburgers. You pick Door #1. Before the contents are revealed to you Monty says "hold on, Jack. Let's look behind door number 3!". The door opens to reveal a hamburger. Then Monty says "OK, do you want to keep your first choice or would you like to switch to door number 2?" What should you do?
Obviously, the thing to do is switch.
There is a 2/3rds chance you picked the wrong door. IF you picked the wrong door, switching will win for you. The only way switching will lose for you is if you picked the right door at the beginning. Therefore, if you switch every time, you will win 2/3rds of the time.
It's easier to picture if you change the total number of choices, but keep the principle the same.
Suppose you have a deck of 52 cards and you have to pick the ace of spades. You select a card, leaving 51 others. Then, 50 cards from that group are revealed, none of which are the ace of spades. Now, which card is more likely to be the ace of spades, the one in your hand or the one that's left?