Imagine that you are participating in a TV show, and there are three doors in front of you. Behind one of them is a brand—new car, behind the other two - nothing, well, or a goat. The presenter, who knows where the prize is, suggests that you first choose (but not open) any door, then opens one of the two remaining ones, behind which there is definitely emptiness, and suggests changing your initial choice. What will you do? According to most people, including mathematicians, the answer is counterintuitive, but probability theory works flawlessly. Now let's explain why.