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?"
And a little fellow shouted: "Why, sir, because yer feet ain't empty."