Gas molecules are in constant motion:
Gas molecules are in constant motion and they move randomly in all directions. These molecules collide with each other and with the walls of the container.

Collisions of gas molecules:
When gas molecules collide with each other, they change their direction and speed. These collisions are elastic, meaning that no energy is lost during these collisions. When gas molecules collide with the walls of the container, they exert a force on the walls.

Pressure exerted by gas:
The pressure exerted by a gas is the force exerted by the gas molecules on the walls of the container per unit area. The pressure of a gas increases with an increase in the number of gas molecules in the container and with an increase in the temperature of the gas.

Gas laws:
The behavior of gases is described by gas laws. The ideal gas law is PV=nRT, where P is pressure, V is volume, n is the number of gas molecules, R is the gas constant, and T is the temperature of the gas. According to this law, the pressure of a gas is directly proportional to the number of gas molecules in the container and the temperature of the gas.

Conclusion:
In conclusion, gas exerts pressure on the walls of a container because of the constant motion of gas molecules and their collisions with each other and the walls of the container. The pressure of a gas is directly proportional to the number of gas molecules in the container and the temperature of the gas.

The particle of gas are continuously moving in all directions into weak forces of attraction. As a result of their random motion they hit each other and also hit the walls of container. This collision of gas molecules are responsible for exerting pressure on the walls of the container.