Our drop is composed of many molecules of water.
They did not all start their descent to earth simultaneously.
Some have been added on the way
and others which were originally together left the road.
Water molecules are continuously added to the drop
from the gaseous state
of water vapour in the atmosphere
while other molecules evaporate from the surface and pass
from the liquid to the gas state.
As a molecule that was initially
inside the drop
came to the
surface this didn’t happen
on its "own initiative".
Some other molecules pushed it
during the incessant
motion, this total "pushing"
that prevails in nature, which
is something we usually forget.
And when another molecule left the surface,
this was because some
of the interior
molecules gave it such a strong "nudge"
that provided the energy needed.
The same goes for condensation.
Collisions of molecules
in the gas
state are those that led a molecule
to pass into the liquid state.
The "crowd" of molecules in a gas may not be
as large as in a liquid,
but the crashes
are again the norm.
A gas molecule under normal conditions "flies"
at the speed of a
very fast airplane,
but it can’t "enjoy" this speed.
The fate of the last seconds
of the molecules
of our drop
is, except for gravity,
determined by collisions.
Collisions with air molecules (which we call friction),
water molecules which
result in addition or removal of material.
What happened then before that?
Exactly the same.
brought a molecule of the ocean
first to the surface and then to evaporation.
led it then, perhaps after months, to the
atmosphere at that point where the drop was formed.