Take a look at your surroundings. You may find a pencil, a book, an apple, a table or a chair. Can you feel the air blowing? These are different forms of matter.
Intensive investigation by scientists over the years led to the development of a mental model (the minuscule particles couldn't be actually seen at that time) known as 'dynamic particle model'.
According to the dynamic particle model all states of matter are made up of tiny particles and matter as a result is an aggregation of very small particles.
All matter consists of atoms and molecules. Each of these atoms and molecules has certain amount of energy.
Depending on the conditions of temperature and pressure, matter can exist in any of the three main states i.e., solid, liquid or gas. Matter can be inter-converted from one state to the other by the addition or removal of heat energy.
The temperature effect on heating a solid varies depending on the nature of the solid and the conditions required in bringing the change.
Pressure has little effect on the state of solids and liquids as the inter-particle distance is already minimized in these situations. However, the effect of pressure in case of gases is considerable.