A teacher was giving a lesson

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

doesn’t run into my feet?”
A little fellow shouted, “It’s because

yer feet ain’t empty.”