The molecular explanation of Boyle's law is simple. The pressure exerted by a gas on the walls of its container arises because the gas molecules strike the walls and rebound (right).

How great the pressure is depends on how fast the molecules are moving, and how often they rebound from the container walls.

The speed of the molecules depends on the temperature and does not affect Boyle's law, which applies only at a constant temperature. But if we squeeze the gas into half its initial volume, then each cubic centimeter of gas has twice as many molecules (below right). Impacts with the walls occur twice as often, so the pressure is twice as great.

Boyle's law is simply a reflection of how often the gas molecules bounce off the walls of the container.