This is a really good question. It reminds me
of an experiment Feynman carried out when he was a
student in college. He asked the question:
you make Jello but NEVER STOP STIRRING IT, WILL IT
I think simple questions like
this show that someone may be headed for a career
in science because they speak of a basic curiosity
about the world and because although EVERYONE uses
Elmers glue all the time FEW PEOPLE stop to ponder
Anyway, I am not totally
certain but I think it has to due with drying.
When you put a layer of Elmers glue on something
it is a THIN layer...and a thin layer has a LOT of
surface area. This surface area allows the glue to
lose water i.e. dry out... and the white stuff
becomes sticky and eventually hardens and bonds.
In a bottle of the stuff, the surface area to
volume ratio is relatively small... and so also
the bottle is closed and so the glue cannot lose
the excess water, and so it stays liquid. A way to
test this is to put some in a dish and see if a
thin SKIN of hardened glue forms. The hypothesis I
have proposed would say that a skin should form at
the top where the glue is adjacent to air... and
to further test you could set up a fan and gently
blow across the surface... the skin should form
faster if I am correct.
So try an
EXPERIMENT. SCIENCE IS THE BELIEF IN THE IGNORANCE
OF EXPERTS...conduct an experiment and let Mother
Nature guide you to the truth!!!!