Little Johnny on Blood Flow

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.." http://www.bassboatcentral.com/smileys/embarassed.gif

"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?" http://www.bassboatcentral.com/smileys/eyes.gif

Little Johnny shouted, http://www.bassboatcentral.com/smileys/glasses.gif

."Cause your feet ain't empty.