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.