Expansion and contraction

Most substances EXPAND on heating and CONTRACT on cooling. This is because heat speeds up molecules, pushing them apart.

Everyday examples:

• Railway tracks have gaps to allow summer expansion (prevents buckling).
• Overhead power lines sag more in summer.
• A tight metal jar lid loosens when warmed under hot water.
• Thermometer liquid rises with heat.

Special case — water: it EXPANDS when freezing (ice floats, less dense than water). That's why ponds freeze on top, keeping fish alive below.