A teacher was giving a lesson on the circulation of the blood. Trying to
make the matter clearer, he said: "Now, boys, 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," said the boys.
"Now," continued the teacher, "what I want to know is this: How is it
that while I am standing upright in the ordinary position the blood
doesn't rush into my feet?"