thermal expansion
Thermal expansion is the tendency of most materials to grow a little bigger when they get hotter and shrink when they cool. It is why bridges and railway tracks have small gaps built into them, why a tight metal jar lid loosens under hot water, and why power lines sag more on a summer day.
When a material warms, its particles vibrate more strongly and, on average, push each other slightly farther apart, so the whole object grows. For a rod, the change in length follows ΔL = alpha L_0 ΔT, where L_0 is the original length, ΔT the temperature change, and alpha the coefficient of linear expansion (a property of the material). For volume, ΔV = beta V_0 ΔT, and for solids beta is very nearly 3 times alpha.
One famous and important exception is water: between 0 C and 4 C it does the opposite, expanding as it cools. This 'anomalous expansion' makes ice less dense than liquid water, so ice floats and ponds freeze from the top down, which is why fish survive winter. Different expansion rates are also put to work in bimetallic strips that bend with temperature to switch thermostats on and off.
The steel spans of a long bridge can lengthen by tens of centimetres between winter and a hot summer, which is why engineers install expansion joints, the sliding gaps you feel your car thump over.
Expansion joints leave room for bridges and rails to grow on hot days.
Water is a famous exception: between 0 C and 4 C it expands as it cools, so ice floats. Without this quirk, lakes would freeze solid from the bottom up.