Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
how does a microwave heat food
Everyone says microwaves cook from the inside out. They do not. The waves only reach the outer few centimetres, and the middle warms later by plain conduction.
Inside the oven a tube called a magnetron makes the microwaves, radio waves twelve point two centimetres long, and a metal channel guides them into the box.
Water molecules are lopsided: one side has a little plus charge, the other a little minus. Fats are lopsided too. That is what the waves grab.
The oven's electric field swings back and forth 2.45 billion times a second, and each swing twists the molecules round. All that jostling is heat.
Food soaks up the waves quickly, so only the outer few centimetres get heated directly. The middle of a thick potato waits for that heat to creep in.
Those waves bounce around the box and leave hot and cold spots, so a turntable moves your plate through them to even things out.
So the whole oven in one picture: a magnetron makes waves, the waves jostle water and fat, the jostling is heat, and it starts at the surface.
Next time the popcorn pops, remember: invisible waves are shaking the water in every kernel.