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Cup of water
When plain water is heated in a microwave in a ceramic or glass container for too long, it can prevent bubbles from forming, which usually help cool the liquid down. The water becomes superheated; when you move the cup, the heat releases violently and erupts boiling water. To avoid this scalding risk, heat water only the minimum amount of time needed or heat it for longer in small cycles.
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That mug you've had for years and years
Certain mugs made before the 1960s, like old versions of Fiestaware, were glazed with materials that could give off radiation and may contain lead and other harmful heavy metals, according to Smithsonian.com. That mug you found at your neighbour's garage sale may look cute, but consider adding it to your shelf collection instead of drinking from it.
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Beware of "microwave safe" labels
The only thing a "microwave safe" label tells you is that you can microwave the container without damaging it or damaging the microwave. In fact, manufacturers aren't even required to test their ceramics to ensure safety after heating, according to the FDA (that's probably why you've burned yourself on too-hot ceramic mugs). Even though something may say it's safe to nuke, if you've never used it before, consider heating your liquid or food on the stove and then transfer it to a bowl, plate, or mug.
Now that you know how to avoid the most common microwave mistakes, read up on the most germ-infested things you touch every day.