A teacher was giving a lesson on the circulation of the blood.

Trying to make the matter clearer, he said, "Now, boys, if I
stood on my head, the blood, as you know, would run into it,
and I would turn red in the face."

"Yes, sir," the boys said.

"Then why is it that while I am standing upright in the ordinary
position, the blood doesn't run into my feet?"

A little fellow shouted, "'Cause yer feet ain't empty."