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

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

"Yes," the class 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."