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A lot of people wonder, “Can you heat plastic containers in the microwave?” The answer to that question is “yes,” so long as you are purchasing microwave-safe plastic food containers. But when it comes to reheating food in plastic food containers with lids, you may not be sure of the best approach to take.

The issue involves the lid itself. You need a sealable plastic food container to keep your food fresh in the fridge or freezer, but when it comes to putting it in your microwave, keeping it sealed is a problem. You need to let the air escape, or you will have a real mess on your hands. Or more specifically, you will have a mess on the walls of your microwave. Obviously that is a situation that you need to avoid.

So your next thought might be to unseal the plastic container, and then rest the lid gently on top. This might be intuitive, but it is also not the best approach. Why? If you do this, there is a chance that your container will end up being vacuum-sealed. At that point, it becomes very hard to get the lid off. You can recognize a container which has become vacuum-sealed on sight, because the lid will usually be indented slightly inward. Getting through the vacuum seal can be impressively difficult.

To understand how this happens, you need to know a little bit about basic physics. Air expands when it is heated—as in the microwave. But when air cools, it contracts. So when you pull your container out of the microwave with the lid on loosely, the contraction of the cooling air inside the container sucks down on the lid, while the air outside the container presses down on it. Additionally, there is action from falling vapor pressure, which enhances this effect.

So ideally, you want to avoid this effect entirely, but you still may want to put a lid on top of your food in the microwave. The best way you can do this is by setting the lid gently on top of the container at an angle so that the corners of the container are left open. This enables the hot air from inside the container to escape, and also ensures that a vacuum seal will not be created.

Of course, you could also simply remove the lid entirely. So long as you are keeping a close eye on your food as you reheat it, you should not end up with a mess. The food will still heat up quickly, so it is not as if this is going to inconvenience you in any significant way.

Now you have some practical knowledge to use in the kitchen. You also know a bit more about thermodynamics, and how some basic physics principles impact your everyday life reheating food in plastic containers. That will give you something interesting to think about while you are waiting for your food to heat up in the microwave!