Just wanted to add another real world example: the coolant system in automobiles does this all the time. That system is essentially a closed loop system. The radiator cap acts as a pressure regulator, and allows fluid to expand into the coolant tank when the pressure rises high enough. The likelihood of pressure is why there are big warnings on your radiator cap telling you to not open it when the engine is hot. If you do, the pressure drops and all that really hot water inside can suddenly decide to turn into really hot steam that will then spray out at you.