or turn the food over, or move it to a different position inside the microwave -- the way microwave works is that it heats up the food unevenly (there's a wave involved).
(I’ve heard the fans that you hear are there to reflect the micro waves and make them bounce all over the place but I don’t know if that’s true. Regardless, most models have a spinning plate which will constantly reposition the food as it cooks.)
Older microwaves had a fan-like metal stirrer inside the cooking box, that would continuously re-randomize where the waves went. This has been out of fashion for several decades.
Composition is part of it, but it isn’t the whole story. A microwave oven is a resonant cavity. There are standing electromagnetic waves in there, in several different modes. They have peaks and nulls. That’s why many microwaves have a rotating plate. It physically moves the food relative to the standing waves.
Like:
1- Put it on the edge of the plate, not in the middle
2- Check every X seconds and give it a stir
3- Don't put metal in
4- Don't put sealed things in
5- Adjust time for wetness
6- Probably don't put in dry things? (I believe you needed water for a microwave to really work? Not sure, haven't tried heating a cup of flour or making a caramel in the microwave)
7- Consider that some things heat weirdly, for example bread heats stupid quick and then turns into stone equally as quick once you take it out.
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