People in prison who have nothing to pass their time tell jokes, and they have memorize it all and numbered it, so one goes:

• 17!
  And they laughed. Another said:
• 10!
  And they laughed. Another said:
• 121!
  And they laughed. But one of them continued to laugh for a very long time, so they asked him why, and he said:
• This is the first time I heard this joke.