Thermal Expansion

Thermal expansion is the tendency of matter to change volume in response to temperature. The mechanism is subtler than "atoms move faster when hot": the interatomic bond potential is not symmetric, so as vibration amplitude grows with temperature, the average separation between atoms increases along with it. A perfectly symmetric potential well would produce faster vibration but no net expansion; real bonds are asymmetric enough that expansion is nearly universal.

Different materials expand at markedly different rates, described by a coefficient of thermal expansion specific to each substance. That mismatch is exploited deliberately in a bimetallic strip — two metals with different expansion coefficients bonded together bend as temperature changes, because one side lengthens more than the other, which is the mechanical basis of old thermostats and some circuit breakers. The same mismatch, unaccounted for, causes trouble: bridges and railway tracks include expansion joints — deliberate gaps — specifically so a hot day's expansion has somewhere to go instead of buckling the structure, and a bathroom tile installed without adequate grout flexibility can crack for exactly this reason.

Water is the well-known exception near its freezing point: it contracts as it cools from higher temperatures like most substances, but between 4°C and 0°C it expands again as it approaches freezing, because hydrogen bonding starts organizing molecules into the more open lattice structure ice will fully adopt. That anomaly is why ice floats, why lakes freeze from the top down rather than the bottom up, and why aquatic life can survive winter beneath a frozen surface instead of the whole body of water solidifying uniformly.