The Monty Hall Problem
"There are three boxes here," the Goddess said. "Only one holds a treasure. Which do you choose?"
You choose the one on the far left.
The Goddess says, "Are you sure you want that one? Let's see what's in another one of the boxes," and opens the box in the middle.
The Goddess says, "You have chosen the leftmost box, but I'll let you change your mind and take the rightmost box instead."
Should you change your mind and take the right box?