Why do birds fly south for the winter?

The other day around dinner, we were asking the kids questions about things they're learning in school. Not sure if the kids are actually studying the migratory habits of birds or not but this is what we learned...

Norm:"Tehani, do you know why birds fly south for the winter?"

After thinking for a really, REALLY long time she replied "Because they can't afford to take the train."

She may not get a good grade in science with an answer like that but her reply had Norm laughing so hard, I thought he was going to fall off his chair.

That's my Hani Bani--if you don't know the answer, make something up!