So far we have regarded each water molecule
as an autonomous entity
its "adventures" undivided.
It may actually be that ever since
the material of the
earth was cooled off,
a water molecule which is then formed
is left intact after four and a half billion
years and merely changed its place.
One time in the atmosphere, another time
in a glacier, and another time in a cell in
the body of a dinosaur.
But it may be that chemistry played a part to.
It may have been split for example during photosynthesis
a large quantity of water each year)
and later have been re-formed by combustion.
Not necessarily from the same atoms which were originally
joined but from
other hydrogen atoms each of which
has its own "history" and an oxygen atom that perhaps
never until now was in a water molecule.
Since chemistry interferes
it would be even more
correct if we consider
the atoms as the fundamental components
of our droplet
we can follow the story back
to the beginning, to the Big Bang.