Campbell's soup containers work in a microwave oven with no sparks -- how?

Question

My mother told me *not* to put anything metal in the microwave oven -- fork, spoon or even a gold plated tea cup. Now Campbell's soup "Ready to Go" product has a zip lid which is removed but leaves a metal lip -- why does this not spark? [I tried the fork and Mom was right!] Here is the soup product ...

http://www.campbellsoup.com/microwavable…

Additional Details

I have seen steel wool (scrub pad) create a microwave fireworks. In fact, the Campbell's lid rim does not respond to a magnet -- telling me it has little iron/steel in these microwavable soup containers.

Answer 1

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Sparks occur when different voltages are present in various places in the metal. The answer about some metals is wrong as all conductive metals will have a problem. But if the shape is carefully controlled, the problem does not exist.

Answer 2

Actually, the only metal products that can not go into the microwave are silver, gold, and aluminum. Steel and stainless steel are fine. Therefore, Campbell's must have made their container out of steel or some cheaper alloy that doesn't have aluminum in it.