A lid is a removable cover for a container, designed to keep things in and other things out. The lid on a jar of peanut butter keeps the contents fresh and prevents spills. A pot lid traps heat and steam while cooking. A shoe box lid keeps dust off the shoes inside.

Lids come in all shapes and sizes, from tiny caps on medicine bottles to massive steel lids on industrial tanks. Some lids screw on with threads, some snap into place, and some just rest on top. The word can also mean an eyelid, the fold of skin that covers and protects your eye.

When someone says to “keep a lid on” something, they mean to keep it secret or under control, like keeping a lid on your excitement before a surprise party. If you flip your lid, you've lost your temper or gotten extremely upset. These expressions come from the idea of a lid containing what's underneath: once the lid comes off, whatever was inside comes pouring out.