Biology lesson

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 positions the blood doesn't run into my feet?"

A little fellow shouted, "Because your feet aren't empty."