Johnny Speaks Out:
A teacher was giving a lesson on the circulation of human blood.
Trying to make the matter clearer, she said, "Now, class, if I stood on my head, the blood would run downward, and my face would then turn red."
"Yes," the class said.
The teacher asked, "Then why is it that while I am standing upright in an ordinary position the blood doesn't run into my feet?"
Little Johnny in the back of the room shouted out, "Cause your feet ain't empty"