Video explains how to solve a brain teaser involving ten people who have been abducted by aliens. The people are arranged single-file, facing the same direction. Each person is wearing either a purple or green hat. No one can see the color of their own hat, but can see the hats of the people in front of them. The aliens ask each person to name the color of the hat they are wearing. If they don't answer correctly, the aliens will displace millions of humans. The problem question is: by what system of rules can you answer the aliens in such a way as to save as many people as possible? [16:58]