thermal expansion
A jar lid stuck tight loosens under hot water, power lines sag on a hot day, and the gaps you hear a train's wheels click over are there on purpose. Almost all materials grow when heated and shrink when cooled. That is thermal expansion, and ignoring it cracks bridges, buckles rails, and shatters glass.
The mechanism lives in the bonding-energy curve. Heat makes atoms vibrate harder about their lattice sites, but the crucial fact is that the bonding-energy well is not symmetric: it is steep on the close-in side (atoms strongly resist being pushed together) and gentle on the far side (they resist being pulled apart less). So as vibration grows, the average atomic spacing shifts outward, and the whole solid swells. For a quantitative handle: a 1-meter steel bar warmed by 100 degrees C grows about 1.2 mm — small, but a bridge is many meters long and a season can swing tens of degrees, so the total movement reaches centimeters.
Strong, deep-welled bonds expand less (ceramics, tungsten) while weak-bonded, low-melting materials expand more (polymers, lead, aluminum). Designers work around it with expansion joints in bridges and rails, allowances in pipelines, and special near-zero-expansion alloys like Invar for precision instruments. The real trouble starts when expansion is blocked or mismatched — then you get thermal stress.
Pour boiling water into a thick ordinary glass and the inner surface heats and expands while the outer wall stays cold and small; the resulting tension can crack it. Thin Pyrex, which expands far less, shrugs the same treatment off.
Uneven expansion between hot and cold regions of one part is a common cause of cracking.
Expansion is driven by the asymmetry of the bond well, not by the atoms themselves getting bigger — the atoms stay the same size, they just sit farther apart on average.