Thermal expansion:                

Thermal expansion is the tendency of matter to change its size when its temperature changes. Most substances expand when heated and contract when cooled.

Key points:

Thermal expansion affects all states of matter: solids, liquids, and gases.

The expansion or contraction depends on the material and the temperature change. Some materials expand more than others.

Thermal expansion can be useful or harmful. For example, it can be used to make thermometers or to create gaps in bridges to allow for expansion and contraction. However, it can also cause problems, like cracks in walls or broken pipes.

Real-world examples:

A hot air balloon rises because the air inside it expands when heated.

A metal bridge expands in the summer heat, which can cause it to buckle if it is not designed to accommodate the expansion.

A thermometer works because the liquid inside it expands when heated.

Remember: Thermal expansion is a natural phenomenon that affects many aspects of our lives. It is important to understand how thermal expansion works so that we can design structures and systems that can withstand the effects of temperature changes.