Answer is: As the pressure decreased in the syringe the volume of the marshmallow increased.
Boyle's Law (the pressure volume law): volume of a given amount of gas held at constant temperature varies inversely with the applied pressure when the temperature and mass are constant.
p₁V₁ = p₂V₂ (the product of the initial volume and pressure is equal to the product of the volume and pressure after a change).
When pressure goes up, volume goes down.
When volume goes up, pressure goes down.
