it's my life

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 should 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?"
Little Johnny shouted, "'Cause yer feet ain't empty."