I created this question I Scream For Ice Cream, with the impression that any metallic object in the microwave will lead to a huge buildup of electric charges, leading to sparking and the potential for the microwave to blow up. Half of the people who attempted the problem felt that the microwave would blow up.
I was surprised (and somewhat horrified) that Michael Mendrin decided that the best way to answer the question was to try it out himself. He made the following observation:
I conducted an experiment and placed a large metal spoon in my microwave and ran it for 30 seconds. It came out warm enough to cut through solid ice cream, and did not cause my microwave to blow up. True, it wasn't a smart thing to do, but science is about repeatability and probability of outcomes, not about a consumer warning of safety about using microwaves ovens.
I then went to research this scenario further, to figure out why "conventional wisdom / old wives tale / consumer warning" seems to contradict the reality of the situation. It turns out that there are numerous factors involved. In particular, because a spoon is rounded, the charges are more evenly spread out, compared to that of a fork, where the charges are concentrated at the tip. Here is a youtube video (skip to 1:30) which shows a fork sparking in the microwave, while the spoon didn't seen to react.
Here are some follow up questions for discussion:
1) What other factors would 'encourage' sparking in a microwave?
2) How long can we leave a spoon in the microwave before something bad happens? (Please don't try this at home.)
3) Why did the metal spoon heat up? 'Conventional wisdom' is that microwaves heat things up by vibrating water molecules.
4) What would you consider to be the correct answer to my originial question? (You can make up your own choices.)