Lock&Lock Classic lids feature four side flaps or "wings" that are hinged and slightly flared at the end for an easy grip when opening. More importantly, they come with holes (rectangular-shaped openings) that line up with matching knobs on the base of the containers, making it easy to latch the lid to the base, locking it into place. This patented blocking hole system maintains the integrity of the lid and helps promote an easy, tightly locked container. OK & Sterilization in hot water OK, Freezer Ok. Made of high quality BPA free plastic. Airtight seal with great elasticity power locks tightly with hollow-center silicon. Microwave, freezer, and top rack dishwasher safe.