Microwaves sparking

[jaysway]

Do microwaves always spark if metal is put inside them?

[WillSmith]

It is possible to put metal in a microwave oven, but it has to be of a special type. The packaging in many ready meals contains a thin film of metal to speed up the cooking process. And a thin metalised film, called a susceptor, is used inside the packaging of microwave meals to convert micrwave energy into heat energy to brown or fry food.

[nickp]

As a ps to the previous answer, it isn't a good idea to put metal into a microwave unless you want to destroy it. The energy generated cannot dissipate and you will destroy the megnetron