Un problema de "huevos":
There was a brillant anthropologists, member of Sigma V, named João. During an expedition to Africa he was captured by a tribe of cannibals and sentenced to serve as a meal. However, the “legislation” of the tribe offered the prisoners a chance to be freed should they be able to overcome a challenge. In the case of João, the challenge was as follows: he would be presented with two chicken eggs, one of them raw and the other one boiled. There would be two boxes. The raw egg would be placed in one box and the boiled egg in the other one. João doesn’t know what the dimensions of the boxes are until he faces the challenge. The walls of the boxes are rigid and opaque. The have the shape of a parallelepiped. One of the boxes has a window in one of its walls. The window is covered by a wire screen whose mesh size João does not know until he faces the challenge. Through the window it it is possible to observe the egg inside the box. The challenge is to find out within 2 minutes which one of the two eggs is raw. It is not allowed to break the eggs, take the eggs out of the boxes or open the boxes. Nothing solid, liquid or gaseous may be put inside the boxes.
João is informed that the challenge would be presented to him after 90 days. Before that, he may count on the help of the villagers to help him work out a solution to the problem. Aside from that, he has the use of all “sophisticated” instruments and everything else that he can find in the village and its surroundings. When the time came for him to face the challenge, at dawn, João was blindfolded and his hands were tied. A few minutes after that, an old villager took an egg, boiled it, dried it, and placed it in a box which he closed. He then took a raw egg and put it in another box, closing the box immediately. The two boxes were placed on a table where they stayed until nightfall. Then João’s hands were untied, the blindfold was removed, he was supplied with the equipment he had requested earlier, and he was taken to the table on which the boxes containing the eggs were lying. He examined them carefully and succeeded in finding out which box contained the raw egg. The challenge was repeated daily for a period of 20 days, each time with different eggs, and every time he was able to identify the raw egg. The admiring cannibals then proceeded to set him free and even gave him lots of jewelry as a gift. How did João manage to save imself?
Yo todavia no se la respuesta, por si alguien pregunta.