Glass lunch boxes are microwave-safe. The lid is the real problem.

Glass lunch boxes are safe to microwave as long as the glass is actually labeled microwave-safe — but the lids are where things usually go wrong, and thermal shock is the thing that’ll crack your container.

Most glass lunch boxes are made from tempered soda-lime glass or borosilicate glass. Borosilicate handles temperature swings much better, but even tempered glass is fine if you’re not dumb about it. The golden rule: don’t move glass between extreme temperatures. Don’t pull a container straight from the freezer and nuke it on high for five minutes. Let it sit on the counter for a few minutes first, or use medium power. And never plunge hot glass into cold water to wash it — that’s the fastest way to shatter it.

The lid is the bigger issue. Most lids are plastic, and many aren’t microwave-safe at all. Even the ones that are should be vented — a sealed lid traps steam, pressure builds up, and suddenly you’re scrubbing soup off the microwave walls. Pop the lid off or crack it open before heating. If you’re reheating fatty foods, do it in short bursts and stir, because fats get hotter than water and can overheat the glass unevenly.

Real talk: if the container says “microwave-safe” and you’re not doing anything extreme with temperature, you’re fine. The glass is the safest part of the whole setup.