4-side of locking system makes it possible to be airtight due to equal pressure to all 4 sides. Strong airtight lock with hollow-center silicone. Airtight seal with great elasticity power locks tightly with hollow-center silicone. Stacking system aims to help space saving. The design of the containers enables excellent space saving in the refrigerator, freezer, cupboard and drawers. So it can be used in the microwave, freezer and dishwasher.