Final answer:
When more gas is pumped into a container, the gas becomes more compressed as its molecules are forced closer together, reducing the volume and increasing the pressure.
Step-by-step explanation:
As you pump more gas into a container, the gas becomes more compressed. Physically, what is happening is that the molecules which are initially at a distance from one another are pushed closer together as pressure increases, thus reducing the volume. This can be visualized by imagining a bicycle pump in action: as air is pumped into a tire, the air molecules are forced into a smaller space within the tire. This is chiefly because gas molecules have a lot of empty space between them, making them highly compressible.
When the volume of a container is decreased by, for example, pushing a piston into a cylinder, the gas molecules inside have less room to move around. This results in the molecules striking the walls of the container more often, thereby increasing the pressure inside the container. If the process continues in a rigid container, this could lead to the container bursting due to high pressure.