Reference to an "empty barrel" brought back fond memories of Professor Julius Sumner-Miller.
http://www.physics.usyd.edu.au/jsm.pdf
I attended his Science School Lectures in the early seventies and can still remember his reaction when holding up an "empty" glass beaker and having us all say "yes" when he asked "Do you agree this beaker is empty?"
Dropping to one knee, he grabbed for his heart and screamed "water, oxygen ..." then stood up calmly and said "There is water vapor in here." He added that there is also oxygen in the air of the beaker as well as Nitrogen, Carbon dioxide ... and proceeded to list 20 or so other gases.
Then explained micro organisms. seed, pollen etc.
By the way that website has some fascinating science puzzles - some of which have appeared on Perplexus - and some more that now probably will!!!!