The built-in water injection feature makes it easy to heat or cool. Simply add hot or cold water into the bottom basin before breakfast lunch or dinner! The bottom bowl is made of food grade PP and can also be used to hold fruits or salads. The lunch box itself does not have a long-term heat preservation effect it needs to be added with hot water to achieve heat preservation. Due to the flexible & rubberized hinge this kids lunch box that is very easy to open and durable even for pre-school children; only need to open 6 clips the buckles are surrounded by soft plastic which will not hurt children's hands.