When the molecules in gas move, they knock at each other but also at the walls of the vessel. In spite of high velocity the knocks of molecules at the wall are weak because the molecules are small. If the number of these knocks per second is high, the forces acting upon the walls can also be great. Because the molecules move in all directions, gas presses on all walls. The pressure in the vessel is thus the same. Therefore pressure is a result of molecule knocks at the walls.
If we increase the number of molecules in the vessel, their knocks are more frequent. Consequently the pressure is greater. But the pressure becomes greater also if we warm gas.
If we reduce the volume of a certain quantity of gas, the frequency of molecule knocks at the walls increases, therefore the pressure increases. And vice versa.
When we press gas, the velocity of molecules increases (because of the motion of the piston knocking at the molecules), which means that also the temperature of gas raises.
