A man is on a game show. He is presented with two doors, one on the left, and one on the right. Behind one is 2 million dollars, and behind the other is a donkey. Choose the correct door to win the prize. There are also two men in front of the doors, and they know which door leads to the millions. One wears a black hat, the other wears a white hat.

The host explains that one of the men is a liar, and will always lie, and the other man will always tell the truth – but you don’t know which is which.

You can ask only one of the men only one question. What is the question, and which man do you ask to ensure you win the money?

A man is the owner of a winery who recently passed away. In his will, he left 21 barrels (seven of which are filled with wine, seven of which are half full, and seven of which are empty) to his three sons. However, the wine and barrels must be split so that each son has the same number of full barrels, the same number of half-full barrels, and the same number of empty barrels. Note that there are no measuring devices handy. How can the barrels and wine be evenly divided?

You are in a cookie factory, and need to make a huge batch of chocolate chip cookies. The recipe calls for exactly 4 cups of sugar. Problem is that you have two buckets. One is 5 cups, the other is 3 cups. Using these buckets, can you measure exactly 4 cups of sugar? How?

A man lives on the fifteenth floor of an apartment building. Every morning he takes the elevator down to the lobby and leaves the building. In the evening, he gets into the elevator, and, if there is someone else in the elevator, or if it was raining that day, he goes back to his floor directly. Otherwise, he goes to the tenth floor and walks up five flights of stairs to his apartment. Can you explain why he does this?