The Blue Forehead Brain Teaser

The blue forehead brain teaser requires no math, just logical reasoning. Here is the problem: There are 100 people in a circle in a room with the lights off. They are told three things: 1. At least one person in the room has their forehead painted blue. 2. Once you deduce that your forehead is painted blue, leave the room. 3. Everyone in the room is a perfect logician. Everyone has their forehead painted, but they cannot ask the color. The lights in the room are turned on for people to look around, then turned off for people to leave or not, then turned back on again, and so on. What happens after this begins, and why?

Key Facts In This Video

  1. After the 100th time the light gets turned on, all the magicians would leave. 01:10

  2. See the logic behind the answer if there were only two people: 04:03

  3. See the logic behind the answer if there were three people: 07:55

Written by Curiosity Staff October 15, 2015

Curiosity uses cookies to improve site performance, for analytics and for advertising. By continuing to use our site, you accept our use of cookies, our Privacy Policy and Terms of Use.