Cold glass is fine. Glass lids are a gamble.

Putting a cold glass lunch box straight from the fridge into the microwave is usually fine, but not all glass lids are made to go in with it.

Most meal-prep glassware is tempered or borosilicate and built for the fridge-to-microwave leap. If yours says “microwave-safe” on the bottom, you’re good. Let it sit on the counter for a few minutes if you want to be extra careful, and don’t blast it on high for ten minutes. Thermal shock is the real enemy — the glass cracks when different parts expand at different rates.

Lids are a different story. A lid can be literally glass and still not microwave-safe. Some have metal rims, some have plastic parts, some aren’t tempered, and some trap steam. If you don’t see the microwave-safe symbol and vent holes, don’t trust it.

If you’re not sure, just microwave the container with the lid off. That solves the whole problem.