The circulation of the blood
A teacher was giving a lesson on the circulation of the blood. Trying to 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 your feet ain't empty."