Here is one of the famous challenging puzzle...

There are 100 prisoners assigned by numbers in 1 to 100 . Any number can be assigned to them . They need not be unique. They can talk one time before they assigned and then don't have any connection. Each one is requested to guess his number (they can use different strategies). He can see their numbers (but not their guess).
How can they do it so at least one of them guesses correctly his number?