This is one of my all-time favorite questions from a student and was asked in complete seriousness.
As we were lining up to go to recess one day, a very bright and gifted third grader asked me, "Mrs. Friedrich, does the stink of a fart follow you when you leave, or does it stay where you left it."
Even better was my answer which explained that a fart was a gas and came out concentrated and then dissipated. He was quite satisfied with the answer. When he walked away, I exploded with laughter.
What's the funniest question you've ever been asked by a student?