A teacher was giving a lesson on the circulation of the 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,sir," the boys 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,"'It's because yer feet ain't empty."