How does a microwave burn food?

Kat Arney put this to Adam Townsend...

亚当-I was unconvinced so this morning, I did a little experiment so I brought a little present into the studio. Andrew is going to take this one and describe this present to the audience.

Andrew - I've got a secure Tupperware tub and inside it is a small piece of whole grain bread. On it, it has a very definite charred mark in the middle like you've held it against a cigarette lighter.

亚当-Can you smell that here?

Kat - I just got a whiff. That's definitely.

Andrew - It's also very solid and hard.

亚当-It is incredibly solid, incredibly hard. That smell though is the same smell that my flat now completely smells of. So don't try this at home. So I put a piece of bread in the microwave for just 2 minutes and after 2 minutes, the middle just started smoking. And so, you get this nice black burn mark.

Kat - What has happened? A microwave does work by making the water in the food vibrate. So how can it do something that seems like a dry fire?

亚当-The first thing that happens here is that all the water gets evaporated off so you get loads of steam. Microwaves are set the frequency that water is very receptive to which means that water heat up. But all of the stuff, in bread in this case, is still getting slightly excited by the microwave energy. So it's just that water is normally more receptive. So you take away the water. What's left while it still tries to absorb some stuff, some of the starch in the bread for example, and so, it also heats up, and ends up stinking up your kitchen.