A teacher was giving a lesson on the circulation of the blood. Trying to make the matter clearer, he said: Now, students, 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 boys said. Then why is it that while I am standing upright in the ordinary position the blood doesnt run into my feet? A little fellow shouted, Its because yer feet aint empty.