Thermal expansion of bodies
When heated, most bodies expand, and when cooled they contract. The reason: as temperature rises, molecules vibrate faster and the average distance between them grows (the molecules themselves do not get bigger). Solids expand least, liquids more, gases most. Engineers take this into account: gaps are left between railway rails, bridges have expansion joints, and power lines sag in summer and are taut in winter. Different metals expand differently, so a bimetallic strip made of two bonded metals bends when heated; it is used in thermostats of irons and heaters and in fire alarms. The liquid in a thermometer (alcohol, mercury) expands with heat and rises. Water is unusual in some cases: from 0 °C to 4 °C it shrinks instead of expanding and is densest at 4 °C (1000 kg/m³), so ice floats on water and the bottom of a lake in winter stays at about 4 °C, so fish survive. Water expands on freezing, which is why a vessel full of water can crack when it freezes.
Experiment: put a balloon over the neck of an empty plastic bottle and place the bottle in a vessel of warm (not boiling!) water — the balloon inflating shows air expanding; then move it into cold water. Safety: use only warm water (about 40 °C), never boiling water or glass bottles, and do it with an adult.