Gases behave in predictable ways. Boyle's law says pressure and volume are inversely related - squeeze a gas, it takes less space. Charles's law says volume and temperature are directly related - heat a gas, it expands.
These laws aren't just theoretical - they explain why balloons pop on hot days and how engines work.
Ideal Gas Law
Put it all together: PV = nRT. Pressure × Volume = moles × R × Temperature. This one equation describes how most gases behave pretty well.