Put a bowl of soup and its ceramic plate through the same two minutes, and only one of them heats itself. That isn't the microwave favoring the soup. Water molecules are polar, with a slightly negative end and a slightly positive end, and the oscillating field inside a microwave has something there to grab. A plain ceramic plate has almost nothing polar in it. Whatever warmth you find in the plate afterward traveled there from the soup sitting on top of it. The field never touched it.
Why the field only grabs onto water
A microwave's magnetron drives an electric field back and forth about 2.45 billion times a second. Water is polar because oxygen holds onto the shared electrons more tightly than each hydrogen does, leaving one end of the molecule slightly negative and the other slightly positive. Flip the field that fast and a water molecule tries to rotate and keep facing it. It can't quite keep up. The drag between all that twisting and its neighbors is friction, and friction shows up as heat. Fats and sugars carry some polarity too, just weaker than water's, so they absorb the field more slowly on their own. Glass, most ceramics, and dry starch have almost no dipole to speak of, and the field passes through them without much to grip. That's the whole reason a dry cracker stays cool in a microwave longer than a damp one does, and the reason your plate is only ever a bystander.
Why fat and sugar run hotter than the water around them
Weaker polarity doesn't mean fat and sugar stay cool. It means they heat more slowly per degree of energy delivered, but they also need far less energy to climb that same degree — fat in particular has a much lower heat capacity than water. Pump the same amount of energy into a pocket of butter or sugar syrup sitting inside something mostly water, and the fat or sugar can race ahead in temperature even though it was slower to start. That's the actual reason the filling of a reheated meat pie or jelly donut can scald the roof of your mouth while the dough around it is still lukewarm. The two ingredients were never heating at the same rate.
Why frozen food thaws unevenly and then runs away with itself
Ice locks water molecules into a rigid crystal lattice, and a molecule that can't rotate can't follow an oscillating field. Solid ice absorbs microwave energy far more poorly than liquid water does — the opposite of what most people assume walking up to the defrost button. The moment one small pocket melts, from a stray hot spot or from residual heat in the packaging, that liquid pocket suddenly absorbs energy far faster than the ice still surrounding it. It can start boiling locally while the rest of the piece is still frozen solid. A defrost setting fights this by cycling the power on and off in short bursts, giving that first melted pocket time to conduct its heat outward into the ice instead of continuing to run away with it.
Why the cavity has cold spots and a turntable exists
The field a magnetron produces at 2.45 gigahertz has a wavelength of roughly 4.8 inches (12.2cm) in open air. Inside a metal box, that wave reflects off the walls and interferes with itself, building a standing pattern of peaks and near-dead zones spaced about 2.4 inches (6cm) apart — half a wavelength. Food sitting in a peak heats fast. Food sitting in a gap barely heats at all, and the two zones can sit inches apart on the same plate. A turntable exists to drag every part of the dish through both zones in turn, averaging out what would otherwise be a scorched ring around a cold center. It isn't a convenience feature. It's the fix for a problem the physics creates.
Why standing time finishes the job the magnetron started
The field only really penetrates about an inch into food before its energy is spent, so anything deeper than that heats by plain conduction — the slow business of one warm molecule handing energy to its cooler neighbor. That conduction doesn't stop when the timer does. Pull a dish out the instant it beeps and the outer layer is still hotter than the center, with heat still moving inward the whole time the dish sits on the counter. Standing time isn't a pause tacked onto the cooking. It's the part of the process where the slow half of the job, the part the field couldn't reach directly, finally catches up.
Why nothing ever browns
Browning is a different chemical process entirely — Maillard reactions and caramelization, and both need a dry surface well above 300°F (150°C) to run at any real speed. A microwave never gets there. Its field pours energy straight into water, and as long as there's any moisture at the surface, evaporation caps that surface at right around 212°F (100°C), the same ceiling an open pot of boiling water can't climb past either. There's no dry, superheated air moving over the food the way a convection oven delivers it, and no hot metal in direct contact with it the way a pan provides. Cut the moisture away entirely and a microwave still won't finish the job, because it never had a way to get a dry surface past that temperature to begin with. If something that came out of a microwave looks browned, another appliance did that part first.