Open vs. Closed Systems
Systems can be either open or closed. A closed system is one where a quantity or series of quantities cannot enter or leave the system. For example, a system might be closed to energy, meaning energy might not be able to enter or leave the system. A vacuum thermos flask does a really good job of stopping energy from leaving the system to keep your drink warm. So it might make sense to treat it as a closed system - but no system in the real world is ever perfectly closed, so it will only be an approximation.