It is possible to demonstrate Boyle’s law with a balloon and an air pump or a cylinder and a plunger. Boyle’s law describes the relationship between the pressure and volume of a contained gas. According to this law, the product of pressure multiplied by the volume of a gas in an enclosed container remains the same if either the pressure or volume changes and the temperature remains constant. If pressure increases, volume decreases.
Reducing the volume of a gas in a container gives the molecules less room to circulate, so they become more densely packed, which increases the pressure against the inner surface of the container. Increasing the volume of the container reduces the pressure since the molecules have more room to circulate and they exert less outward pressure from inside the container.
A plunger and a cylinder filled with colored smoke can demonstrate this effect. Decreasing the volume by pushing the plunger downward forces the molecules closer and increases the pressure, which is visible by the color of the gas intensifying. Increasing the volume by pulling the plunger out and reducing the pressure causes the color to become less intense since the molecules have more room to circulate and spread out.
The same effect is visible by using a bicycle pump and a balloon. Pump enough air into the balloon to make it expand slightly, and then decrease the volume by squeezing or twisting the balloon in half. After this, you can see the resulting increase in pressure because the air molecules in the balloon have less room to circulate and get forced to the tips, which causes the skin to bulge or stretch.