Don't snap the lid shut before freezing

Don’t snap the lid shut before freezing.

You can freeze a glass or ceramic lunch box with the lid on, but the trick is to leave the lid loose until the food is solid.

The main culprit here is water expanding by about 9% when it turns to ice. If you fill the container and seal it airtight, the expanding liquid has nowhere to go. That pressure pops the lid off violently or, if the lid is strong enough to hold, transfers all that force to the glass or ceramic and cracks it.

Sudden temperature change is a separate issue. Never put a warm glass container straight into the freezer—thermal shock can shatter it before the ice even has a chance to expand. Let your food cool to room temperature first. The same goes for taking a frozen glass dish and running hot water on it to thaw; that’s a great way to explode it.

As for the seal itself, an airtight plastic lid that stays snapped through the freezing process is going to suffer. The combination of cold making the plastic brittle and the internal pressure pushing outward can permanently warp the lid, ruining the seal for future use. Leave it cracked open for the first few hours, or just rest the lid on top without locking it. Once the contents are frozen solid, snap it shut for storage.

Leave a little headspace in the container, too. It gives the ice room to expand without touching the lid at all.