Calvin and Hobbes by Bill Watterson for July 21, 1987
Calvin: Hey Dad, what are clouds made of? Dad: Hmm... I used to know that. I think they're mostly water. Calvin: So how come they float? Dad: Well, it's sort of ecaporated water. Maybe there are some other gases, too. I'm not sure. Calvin: So why are they white when the rest of the sky is blue? Calvin: Heck, beats me. I guess we ought to look this stuff up. Calvin: I take it there's no qualifying exam to be a dad.