Kitchen3 min

How does a microwave heat food?

Ever had leftovers come out like lava at the edges and still cold in the middle? Here’s what’s going on in there.

Heating time1:00

Where to lookEdge

In the closer look

The wave twists the water.

The wave is still strong here. It twists the water molecules one way, then the other, all in step. They knock into their neighbors, and all that jostling turns into heat.

After 1:00, the edge is warm and the middle is fridge cold.

The molecules are slowed way down here. Really, they twist back and forth billions of times a second. The heat readings come from a simple model of a slice about 2½ inches thick. Real food and ovens vary.

Why water?

A water molecule is a little lopsided. Its big oxygen end is slightly negative. Its two small hydrogen ends are slightly positive.

The wave pushes on one end and pulls on the other, so the molecule turns. Then the wave flips and turns it back. Food is full of water, so the whole outside of the lasagna fills with molecules twisting billions of times a second. They bump into everything around them, and all that jostling turns into heat.

Dry things warm up a lot less. An empty plate often stays fairly cool. When a plate does get hot, most of that heat usually came from the food sitting on it.

The same molecule, turned one way, then the other.

Wait, what?

It doesn’t cook from the inside out.

A lot of people think microwaves heat food from the middle. It’s the other way around. The waves fade as they go in, so most of the heating happens in the outer inch or so. The middle has to wait for that heat to creep inward.

Why stirring and waiting help.

Heat moves through food slowly. Stirring soup brings hot food from the edge into the middle. With something you can’t stir, like lasagna, letting it sit for a minute or two gives the heat time to spread inward. That’s why so many packages say “let stand” after heating.

Put a name to the waves

Microwave

An invisible kind of wave, a cousin of radio waves and light. The oven is named after them.

“The microwaves mostly heat the outside of the lasagna.”

Go down the rabbit hole

ReadRead the FDA’s plain answerThe US Food and Drug Administration explains how microwave ovens heat food, and says outright that they don’t cook from the inside out.ReadRead why water molecules flipA short answer from McGill University’s Office for Science and Society on why water’s lopsided charge makes it twist, and how that turns into heat.ReadRead why packages say “let stand”BBC Science Focus on how far the waves reach into food, and how the heat gets to the middle afterward.

TryFind the cold spot in the drawingDrag Heating time all the way to 3:00. Then slide Where to look from Edge to Middle. In the closer look, the molecules almost stop twisting, and Food here drops from Steaming to Cool.

TryCheck the middle next timeNext time you microwave something thick, like leftovers, stop halfway and taste a bite from the middle. It’s often still cool while the edges bubble.