Air Mail

It took the four-year-old to notice this one.

"Ummm. Dad?"

"Yeah bud?"

"Why ... um ... why ...

why is that mail box up there on that pole?"

"What!?"

"Up there ... LOOK! That pole over there."

"Oh. That's for air mail."

"What's air mail?"

How do you explain air mail to a four-year old?

Or that people in that Dam place just have a strange sense of humor?