It doesn't "add heat" from the outside the way a stove does. A microwave floods your food with an invisible, fast-reversing electric field — and the water already inside the food does the rest. Here's the one idea that explains all of it.
A water molecule is lopsided: one end is slightly positive, the other slightly negative. That makes it a tiny electric compass needle — point an electric field at it and it swings to line up, just as a compass needle swings toward north.
A microwave's job is simple: it fills the oven with an electric field that flips direction about 2.45 billion times a second. Every flip, the billions of water molecules in your food scramble to re-align — twisting back and forth furiously.
That frantic twisting is heat. Hotter just means molecules jostling faster, and here we're driving the jostle directly. Keep this picture — a field that flips, and water that chases it — and the rest of the page is just consequences.
The real field reverses billions of times a second — far too fast to picture. Slow it right down and watch a water molecule chase the arrow. Speed it up and the chasing turns into a blur of twisting. That blur is the heat.
Drag the speed up. Notice the molecule never catches the field — it's always swinging to catch up, and that constant struggle is what warms the water.
People picture microwaves zapping the center of the food first. The opposite is closer to the truth. The waves are strongest where they enter and get absorbed as they go, so the outer layer soaks up the most energy. By a couple of centimeters deep, much of the wave is already spent.
The depth where the wave has faded to about a third of its strength is roughly 1–2 cm in wet food. That's why a thick burrito can be lava on the outside and icy in the middle: the microwave never reached the middle — conduction (slow heat creep) and a standing rest have to finish the job.
"Microwaves cook from the inside out." They don't. Energy is deposited near the surface first and weakens with depth. The myth comes from the real fact that microwaves heat the water throughout a thin food evenly, instead of searing only the outside like a pan — but for anything thick, the center is the last place to warm up.
2.45 GHz isn't a magic "resonance" of water — it's just one of the bands regulators set aside for ovens. Water absorbs it well enough to heat efficiently but weakly enough that the waves still sink a few centimetres in instead of being soaked up entirely at the surface — while glass, ceramic, and most plastics barely absorb it at all, so the food heats while the plate stays cool (the plate only warms by touching hot food). Fats and sugars also absorb somewhat, which is why they can get scorching. Metal is a different story: it reflects the waves and concentrates the field at sharp edges, which is what makes sparks.
Inside the metal box, microwaves bounce off the walls and overlap, building a fixed pattern of strong and weak zones — a "standing wave." Food sitting still cooks unevenly. Toggle the turntable and the half-time stir to see why both exist.
The bright bands are the standing-wave hot spots. Run it raw, then switch on the turntable, then add a mid-cook stir, and watch how even the final temperature gets.
The hot/cold pattern is roughly fixed in space. A turntable moves the food through that pattern so every part spends time in a hot band — it averages the heating over a ring, but the dead center of the plate barely moves, which is why the middle can still lag. A stir (or flipping/repositioning) physically relocates the cold food into hot zones and mixes warm and cold together, which is the most effective fix of all. Letting the food stand after also lets conduction even things out.
No grade, just a gut-check on the mental model. Pick an answer to see why it's right or wrong.