Most metals expand when they're heated and contract when they're cooled. The amount of expansion or contraction depends on the metal's thermal expansion coefficient, which is a measure of how much heat energy it can absorb and then release over time
As a result, metals are good at doing just that: restraining their natural tendency toward expansion (because it doesn't like the heat)〞as well as their tendency toward contraction (because it doesn't like the cold)
This means that metals can be used to create springs with a variable stiffness by changing the metal's temperature.
forged metal fidgets