Q: Why was it so hot in the stadium after the baseball game?
A: All the fans left!

Explanation: At a baseball game, the fans are the people who are watching the game.  The fans leave the stadium when the game is over. Baseball games are played at a stadium.  A fan is also a small (or large) machine that has a part that spins to move air.  If there are no fans (that move air) at a stadium the stadium could get hot.

This joke plays on the double meaning of the word fan- and that is why it is funny!

Go Cubs!