You participate in a game show where you must choose between 3 doors. Behind 2 doors is a goat, but behind 1 door is a car. You pick a door, say door no. 1, but before you open the door, the host opens another door, say door no. 3, which has the car behind it. "Oops," says the host.
You lose.
That's why in Who Wants To Be A Millionaire if you're unsure you should always change your answer after the 50/50.
Your first choice is 1/4 chance to be right, but if the 50/50 eliminates two of the other choices the remaining one is 3/4 chance to be right.
/s