Empty Head!

A teacher was giving a lesson on the circulation of blood. Trying to make the matter clearer, he said, "Now class, if I stood on my head, the blood as you know, would run into it and I should turn red in the face." "Yes, sir," the class said. "Then why is it that while I am standing upright in an ordinary position the blood doesn't run into my feet?" Pappu shouted, "Cause your feet ain't empty."