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It’s never a good idea to put fruit in the microwave. Grapes, for example, will explode, and raisins send off so much plasma that they can ruin your microwave.
Hot peppers, especially those of the dried variety, can be a very unpleasant thing to try to microwave. The capsaicin, or the active ingredient in making peppers spicy, vaporizes when exposed to microwaves, and getting a faceful of that vapor makes for a highly uncomfortable experience.
Thought that you could hard-boil an egg by microwaving it? Think again. The insides will get super-hot, and the steam will have nowhere to go, so you’ll end up scrubbing chunks of yolk off of the inside of your microwave for hours.
You’re hopefully aware of this, but metal really shouldn’t go in a microwave. Even the small amount found in a twist tie can send supercharged plasma shooting through your appliance, and aluminum foil can become an open flame. Place it on a microwave-safe dish instead.
Don’t try to boil a mug of water in the microwave. The water superheats but is unable to physically boil because the vessel remains cool, so when you try to stir it or add a teabag it boils all at once, exploding upwards, right in the direction of your face. If you must microwave water, stick a stirrer or something in it so it can boil as usual.
If you put bread in a microwave you won’t end up with a ruined microwave, just a ruined piece of bread. After more than 10 or so seconds in a microwave, enough moisture leaves the bread to make it a hard, stale, chewy mess. The best way to bring back stale bread is to stick it in the oven for a few minutes.
Some plastic is microwave-safe, but it’s always labeled accordingly if it is. If it isn’t, it will melt inside your microwave, making for a nearly-impossible cleanup.
It's true that if you put a sponge in the microwave for 30 seconds or so, it'll kill off most of the bacteria on it. But the cons outweigh the pros here: Your kitchen will end up smelling awful. Sponges are cheap; just buy a new one.