A teacher was giving a lesson on blood circulation. Trying to make the matter clearer, she said: "Now, 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?"
One loyal little charge shouted: "Cause your feet ain't empty."