blood.
Trying to make the matter clearer,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 in the ordinary position the blood doesn't run into my feet?" A little
fellow shouted,
"Cause your feet ain't empty."