Circulatory Lesson

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A teacher was giving a lesson on the circulation of the blood.

Trying make the matter clearer, she said, "Now, class, if I stood on my head, the blood, as you know, would run into it, and I would turn red in the face."

"Yes," the class 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, "Cause yer feet ain't empty."