A microwave oven heats food in a completely different way from a flame or a hot ring. It fills the inside with an invisible wave that flips back and forth incredibly fast, about 2.45 billion times every second. Now, water molecules are a bit like tiny magnets: they have a positive end and a negative end. So when the field flips, the water molecules try to flip with it, twisting one way then the other, over and over, frantically. All that twisting makes them rub and jostle against their neighbours, and rubbing makes heat, the same way your hands warm up when you rub them together. Since most food is full of water, it heats up fast. Fat and sugar do not twist as well, so they warm more slowly. And here is the strange part: ice barely heats at all, because its water molecules are frozen in place and cannot twist. Try the different foods in the simulator and watch the molecules.
Most people think a microwave heats food from the outside in, like a tiny oven. In fact it makes the water molecules throughout the food twist and rub, so it heats wherever there is water at once.
What's actually happening
The microwave oven is one of the few genuinely alien pieces of technology sitting in an ordinary kitchen. Every other way we cook is, at heart, the same trick our ancestors used: put food next to something hot and let the heat soak inward. A flame, a hotplate, an oven, a grill, all heat the outside of the food first, and the warmth conducts slowly toward the middle. The microwave does nothing of the sort. It reaches into the food and stirs the water itself.
The heart of the machine is a device called a magnetron, which produces an electromagnetic field oscillating at 2.45 gigahertz. That number is the key to everything: it means the field reverses its direction about 2.45 billion times every single second. To see why that matters, you have to know one fact about water. A water molecule is polar, meaning it has a slightly positive end and a slightly negative end, like a minuscule bar magnet. Drop a polar molecule into a flipping electric field and it will try to line up with it, turning to face one way, then whipping around to face the other as the field reverses, billions of times a second. The molecules are not being warmed from outside; they are being driven to twist furiously in place.
And twisting furiously in a crowd means friction. Each frantic water molecule jostles and rubs against its neighbours, and that rubbing is exactly what heat is at the molecular level. So the food cooks from within, wherever there is water, which is why a microwave warms a bowl of soup so much faster than an oven. This also explains two everyday puzzles. Fat and sugar are far less polar than water, so they do not follow the field as eagerly and heat up more slowly, which is why a jam doughnut can have molten jam inside and a barely warm exterior. And ice, strangely, barely heats at all: in a crystal the water molecules are locked in place and cannot twist, so a frozen lump just sits there. That is why microwaves have a defrost setting that pulses the power, giving the melted edges time to spread warmth to the still-frozen middle. One last detail explains the spinning plate: the field does not fill the oven evenly. It forms a standing wave with fixed hot and cold spots, so without a turntable to keep the food moving, you would get scalding patches next to cold ones.
A microwave heats food by flipping polar water molecules billions of times a second, and the friction of that twisting is the heat.
- 1Spread a thin, even layer of grated cheese on a microwave-safe plate and take out the turntable (or stop it).
- 2Heat for a short burst and look: you will see melted patches with unmelted gaps between them, the hot and cold spots of the standing wave.
- 3Put the turntable back and repeat, the spinning smears those spots into an even melt, which is exactly why the plate rotates.
Common questions
Because water molecules are polar, like tiny magnets, and twist to follow the rapidly flipping field. That frantic twisting causes friction with neighbouring molecules, and friction is heat. Since most food is full of water, it warms quickly and from within.
In ice the water molecules are locked into a crystal lattice and cannot rotate freely, so they can hardly respond to the flipping field. With little twisting there is little friction, so frozen water absorbs very little energy until it begins to melt.
The field forms a standing wave inside the oven, with fixed hot spots and cold spots. Spinning the food on a turntable moves it through those spots so it heats more evenly instead of having scalding patches next to cold ones.