儿童笑话|Blood Circulation

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^{1 upright in an ordinary position the blood doesn't run into my feet?
A class member shouted, 'Cause your feet ain't empty.}