Microwaves do not cook from the inside out

The points below about how microwave ovens heat food are ordered by how much difference they make, not by how often they get repeated.

What the oven actually does

The oven fills its cavity with electromagnetic radiation that causes water molecules in food to rotate rapidly, generating heat through friction. That radiation is absorbed as it travels, so its intensity falls off steeply with depth into the food.

In most foods the useful penetration is on the order of a couple of centimetres, after which very little energy remains. The outer region therefore heats directly and the interior heats by ordinary conduction from that region inwards. This is the opposite of the popular description, and it explains most of the frustrations people have with the appliance.

The resonance myth inside the myth

A common secondary claim is that the frequency used is tuned to a natural resonance of the water molecule. That is not the case, and if it were, the energy would be absorbed entirely in the first millimetre and nothing would cook. The frequency was chosen from bands allocated for industrial and domestic use, balancing penetration against absorption.

Water absorbs across a broad range rather than at one sharp frequency in this context. The resonance story appears in a great many otherwise reliable explanations, which makes it worth flagging.

Why the middle of things is cold

A large item heats at the edges and relies on conduction inwards, and conduction through food is slow. Turning the power down and extending the time gives conduction more opportunity to even things out. Standing time after heating is part of the process rather than an afterthought, since the temperature continues to equalise.

Somewhere in the retelling, stirring partway through does the same job much faster for anything liquid or loose. The instruction to stand and stir is on the packet because the physics requires it, not because the manufacturer is being cautious.

Why some things get dangerously hot

Foods differ enormously in how much they absorb, depending mainly on water content, salt, sugar and fat. A jam filling in a pastry absorbs far more than the dough around it, and can reach a very high temperature while the outside feels warm. That mismatch is where the inside-out impression comes from, and it is about composition rather than about depth.

Trace it back and sugar and fat can exceed the boiling point of water, which is why fillings and oils can be startlingly hot. Cutting into anything filled before eating is the practical response, and it is more useful than any timing rule.

The cold spots and the turntable

Radiation in the cavity forms a standing wave pattern with regions of higher and lower intensity. The turntable exists to move food through that pattern so no part sits permanently in a low-intensity region.

Some ovens use a rotating deflector instead, which stirs the field rather than the food. Neither approach is perfect, which is why uneven heating persists even in good machines. The classic demonstration involves removing the turntable and heating a flat layer of something meltable to reveal the pattern.

Printing a correction has a poor record of taking a myth out of circulation.

What the appliance is genuinely good at

It heats water-containing food quickly with high energy efficiency, because the energy goes into the food rather than into the oven. It is poor at browning, since the surface stays near the boiling point of water and browning reactions need considerably more.

Trace it back and combination appliances add a grill or hot air precisely to supply the heat the microwave cannot. Reheating is where it excels, and the results improve substantially with lower power settings and longer times. Most complaints about it are complaints about conduction, which no oven of any kind can hurry.

Everything above, in order of what to do first

1. What the oven actually does. The oven fills its cavity with electromagnetic radiation that causes water molecules in food to rotate rapidly, generating heat through friction.
2. The resonance myth inside the myth. A common secondary claim is that the frequency used is tuned to a natural resonance of the water molecule.
3. Why the middle of things is cold. A large item heats at the edges and relies on conduction inwards, and conduction through food is slow.
4. Why some things get dangerously hot. Foods differ enormously in how much they absorb, depending mainly on water content, salt, sugar and fat.
5. The cold spots and the turntable. Radiation in the cavity forms a standing wave pattern with regions of higher and lower intensity.
6. What the appliance is genuinely good at. It heats water-containing food quickly with high energy efficiency, because the energy goes into the food rather than into the oven.

The takeaway

It heats a shell a couple of centimetres thick, and everything after that is the same slow conduction as any other oven.

Believing it was ordinary. Continuing to is the avoidable part.