Using a hand-operated vacuum pump, we can demonstrate the relationship between pressure and volume of a gas. According to Boyle's law, the pressure of a gas of constant mass and temperature will be inversely proportional to its volume.
In our demonstration, we will reduce the ambient pressure within the sealed container, hence allowing the higher internal pressure of a balloon to cause it to expand. When the volume within the balloon increases, the internal pressure can be observed to decrease until it is in equilibrium with the surrounding pressure.
While the relationship between pressure and volume is not exactly obeying Boyle's law due to additional factors such as the tension due to the elastic property of the balloon, it does demonstrate an inverse relationship.