Q: Why do we put candles on top of a birthday cake?

A: Because it’s too hard to put them on the bottom!

Explanation: Well, this is funny because the listener is expecting a real explanation that has to do with adding candles to a cake, perhaps even explaining why we have cake for birthdays.  But that is not the answer that the listener hears.  The answer is just an obviously funny one!  Candles on the bottom of a cake!

Happy Birthday, Ana Maria!