Do microwaves cook food from the inside out?

No. Microwaves don't cook food from the inside out. They heat water molecules mainly in the outer couple of centimetres; the middle of thick food warms only as heat spreads inward, which is why leftovers can stay cold in the centre.

What's actually true

Microwaves don't cook food from the inside out. Microwaves make water molecules in food vibrate, which creates heat inside the food. But the waves mostly reach only the outer couple of centimetres. In anything thick, the middle warms up only as heat spreads inward, which is why the centre of leftovers can stay cold.

Takeaway: Stir halfway and let food stand a minute so heat can spread to the middle.

Why people believe it

Microwaves feel mysterious because the oven and the dish stay cool while the food gets hot, unlike a stove that heats from the outside. Since the heat seems to appear inside the food, people extend that to 'it starts in the middle'. Some foods add to the confusion: a pastry or a potato can feel cooler on its dry surface than in its moist filling, which heats well. The waves still reach only the outer layer first.

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