Pressure is directly tied to the microscopic behavior of matter. Gas molecules collide with one another continuously. The mass of any single molecule is extremely small, but they are so numerous that collisions with the walls exert a force that pushes on those walls. Pressure is the expression of this force per unit of surface area.
The smaller the volume, the more numerous are these collisions, and the greater the resulting pressure is.
Click and drag the piston to modify the pressure.