Boyle's law
Block the end of a bicycle pump with your thumb and push the handle in: the further you squeeze, the harder the trapped air pushes back. Boyle's law is the precise version of that everyday feeling — for a fixed amount of gas at a fixed temperature, squeezing it into less space raises its pressure.
Stated exactly, pressure and volume are inversely proportional: their product P times V stays constant. Halve the volume and the pressure doubles; triple the volume and the pressure falls to a third. Kinetic theory explains it simply — crowd the same molecules into less room and they strike the walls more often, so the pressure climbs.
Boyle's law, found in the seventeenth century, was among the first quantitative laws of chemistry and is one of the special cases hidden inside the ideal gas law. It matters as a clean, memorable rule for gas compression — used in everything from scuba diving safety to designing pumps and syringes.
A scuba diver's lungful of air doubles in volume as they rise from ten metres deep to the surface, where the pressure has halved — which is exactly why divers are taught never to hold their breath while ascending.
Halve the pressure and the gas doubles in volume — Boyle's law in the sea.
The law holds only at constant temperature and constant amount of gas. Warm the gas while squeezing it and the simple inverse relationship no longer applies — you would then be combining Boyle's law with Charles's.