A teacher was giving a lesson on the circulation of 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," the pupils said.

"Then why is it that while I am standing upright the blood doesn't run into my feet?"

A little boy shouted, "It's because your feet aren't empty."