A teacher was giving a lesson on the circulation of 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 would 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 doesn’t run into my feet?”
A little fellow shouted, “It’s because your feet ain’t empty. “
This joke is included in the Black Book of Jokes. You can order it from here.