### Theory:

1. Expansion:
When we heat the substance, it expands because the increase in the molecule's energy creates more vibrations and movements, usually creating more distance between themselves. Similarly, it contracts when cooled.
The expansion of a substance on heating is called the thermal expansion of that substance.

The change in length, area, or volume due to contraction or expansion is directly proportional to temperature change.
Example:
We can see an everyday example of thermal expansion in the kitchen.
Almost everyone has had the experience of trying unsuccessfully to open a tight metal lid on a glass container.

Did you have that experience? What did you do to open it?

Most of you can open it after running hot water over the lid; it gives way and opens at last.

Reason:
When the hot water is poured over the lid, which is made of steel, having a high coefficient of expansion starts to expand.

On the other hand, glass with a low coefficient of expansion is relatively low expansion compared to the lid. This lower expansion in glass compared to steel helps to open the container.

Expansion in Solids:
Whenever we heat a solid, there is an increase in the dimensions of the body. It is known as expansion in solids.

It undergoes three types of expansions.

1. Linear or Longitudinal expansions
2. Areal or Superficial expansions
3. Cubical or Volumetric expansions

1. Linear expansions: When we heat a solid, if there is an increase in the body's length, this is called Linear expansion. It is also known as Longitudinal expansion.

2. Areal expansions: When we heat a solid, if there is an increase in the area of the body, this is called Areal expansion. It is also known as Superficial expansion.

3. Cubical expansions: When we heat a solid, if there is an increase in the volume of the body, this is called Cubical expansion. It is also known as Volumetric expansion.