Quote from Lobster:
A monastery houses 100 monks who are not allowed to communicate with each other in any way. They meet only once per day, for dinner at a large round table, but even then they are not allowed to talk, give signs or anything, only to look at each other. There is only one exception to this rule: In case of an emergency the head of the monastery is allowed to speak. One evening he does that and tells his monks that there is a disease among them. Those who have contracted it have a big red spot on their forhead. Although the disease is not contagious, he gives the order that every monk has to kill himself the day he finds out that he has the disease. There are no mirrors in the monastery and no way for any monk to see his own forhead. But every evening each one sees all the other monks' forheads, just not his own. Ten of the 100 monks have the disease and therefore the dot. After the first supper, they all go to their rooms, and everyone returns the next evening. The same thing happens until the tenth day after the announcement. On the eleventh evening only 90 monks return from their chambers to eat. The other 10 have committed suicide. They were exactly the diseased ones. How did they know?