Every night at bed time Arianna and I play a game she calls the "planet test."  I ask her questions about the planets and she answers them.  (I have had to do some research to keep her challenged.)  Tonight one of the questions was "Which planets are gas giants?"  She names, of course, Jupiter, Saturn, Uranus and Neptune.  Then I asked, "Why are they called gas giants?"  "Because they fart?" she says, and then breaks into giggles.  I suppose I asked for it.