A blonde teacher was giving a lesson on blood circulation... Trying
to make the matter clear, 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 the blood doesn't
run into my feet?"

The answer came from the back of the class, "Cause your feet ain't
empty."