A lesson about blood flow and circulation

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."

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