You need to put a metal rod
into a hole in a metal cylinder.
it is too tight. which would
be the best strategy to make
the rod fit? (use cooling and/or heating)

-- Put the rod into the freezer for a while.  As it cools,
it contracts (gets smaller) slightly.

-- Put the cylinder into hot hot water for a while.  As it heats,
it expands (gets bigger) slightly.

-- Bring the rod and the cylinder togther quickly, before the
rod has a chance to warm up or the cylinder has a chance
to cool off.

-- I bet it'll fit now.

-- But be careful . . . get the rod exactly where you want it as fast
as you can.  Once both pieces come back to the same temperature,
and the rod expands a little and the cylinder contracts a little, the fit
will be so tight that you'll probably never get them apart again, or even
move the rod.

The answer is water molecules are split.cellular respiration and photosynthesis are complement  reactions to each other. these are reverse reactions. in cellular respiration glucose and oxygen yield water and carbon dioxide. so, when respiration begins,  glucose molecules are split. in the opposite reaction - in photosynthesis, carbon dioxide and water yield glucose and oxygen. thus,  when respiration begins, water molecules are split.