Hundred most brilliant logicians are handpicked from the world and invited to a room. But before they could enter, they are told that at least one of them has a black forehead. Whenever anyone can frame out that he is having a black forehead, he needs to leave the room when the lights are turned off. After that, the lights are turned back on and those who could infer that their forehead was black have left the room.
What happens if they have painted every forehead black?