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.""