Boyle's Law with shaving cream. Footage of shaving cream in a beaker inside a sealed container attached to a pump that is evacuating the air. As the pressure drops inside the container, the shaving cream expands. This is caused by air within the shaving cream expanding as the pressure drops. When air is let back into the container, the shaving cream collapses. This is an example of Boyle's Law, a gas law that states that the pressure of a gas is inversely proportional to its volume.