Thermal expansion of objects
Heat passes by itself only from a warmer body to a cooler one. All bodies are made of particles that are always moving. When a body is heated its particles move faster and the distances between them grow, so the body expands; when cooled the distances shrink and it contracts. Usually gases expand most, liquids less and solids least. A thermometer works by the expansion of its liquid (alcohol). Engineers allow for this: small gaps are left between railway rails and bridge slabs, otherwise they buckle in summer; power lines hang loose in summer and are taut in winter. If boiling water is poured into a cold glass its inner side expands quickly while the outer side has not yet expanded, so the glass may crack. Water is an exception: from 0 °C to 4 °C it contracts when warmed, so it is densest at 4 °C and ice floats on water. Safety: do expansion experiments only with warm water (up to about 50–60 °C) and with adult supervision.
Expansion of a liquid: fill a small bottle with coloured water, seal it with a stopper or plasticine fitted with a thin tube (straw) so the water shows in the tube. Stand the bottle in a bowl of warm water (not above 50 °C) and watch the level in the tube rise, then fall when it is put into cold water. Safety: do not use boiling water; an adult pours the warm water; a plastic bottle is preferable to glass; wipe up spills.