**Sent by Xavier Cases Camats.**

**11.-** The warden of a prison lets know that he will release somebody randomly to celebrate he has been warden for 25 years.

A man is chosen and told he will be released if he takes a white ball from a box, where there are 9 black ones and just 1 white ball.

The prisoner knows thanks to a tip-off that the warden will make all the balls be black. The next day, the game takes place and the prisoner is released.

How could he get out of prison if every ball is black?

-- Solution of the riddle --