thermal expansion
A glass jar with a stuck metal lid often pops open after you run it under hot water. The trick works because the metal lid swells a touch more than the glass when warmed, loosening its grip. This everyday swelling of solids as they get hotter is thermal expansion.
Heat makes atoms vibrate harder, but here is the crucial twist: the bonds between atoms are slightly lopsided springs, easier to stretch than to compress. So as an atom vibrates more vigorously, it does not just swing symmetrically about its home point — on average it sits a little farther from its neighbour than before. Multiply that tiny extra gap across billions of bonds and the whole object grows measurably larger. Heat it more, the atoms shift farther out, and it expands further still.
Thermal expansion matters in everything from the gaps in railway tracks to the design of thermometers and the dental fillings that must not crack as they warm. The honest caveat is that the cause is not the vibration itself but its asymmetry, the anharmonicity of the bonds: a hypothetical crystal with perfectly even springs would vibrate when heated yet never change size at all. Some real materials, like water near freezing, even buck the trend and shrink over certain ranges.
The Eiffel Tower is about 15 centimetres taller on a hot summer day than on a cold winter one. Its iron quietly expands and contracts with the seasons — a giant, slow demonstration of billions of slightly-stretched atomic bonds adding up.
The Eiffel Tower grows in summer and shrinks in winter by about a hand's width.
Different materials expand by different amounts, and that mismatch is itself useful: a bimetallic strip glued from two metals curls as it heats, which is how many old thermostats sense temperature and switch a furnace on or off.