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 fastas you can.  Once both pieces come back to the same temperature,and the rod expands a little and the cylinder contracts a little, the fitwill be so tight that you'll probably never get them apart again, or evenmove the rod.