In this puzzle, we ask you to use logic to help each boy figure out the color of his hat.

Lucy had four hats, two red and two blue. She showed them to her three friends, then made them stand one behind another and shut their eyes.

Lucy put a hat on each of their heads. Then she hid behind a wall and put on the last hat. “To keep all the hats, just one of you must tell me what color hat you are wearing. You can open your eyes, but you must not turn around and look at the boy behind you,” she said. "Only one person should call out, and no one must talk apart from the person calling out the color of his own hat."

Each boy could see only the color of the hat or hats in front of him. How could each be sure to guess the color of his own hat correctly?