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.""