I heated up some pinto beans today for lunch in the microwave. When I took the bowl out and set it down, I realized that there was a spoon in the bowl from the last time I had eaten from that bowl. Why didn’t anything spark? Do pinto beans have secret electrochemical properties that shielded the spoon? Or (more likely) did I not have the bowl in the microwave long enough? It wasn’t that long, but it was certainly more than just a few seconds.
I dunno, but years ago a friend of mine (really, really, it wasn’t me, I swear) re-heated a hamburger in one, and the one billionth of foil that was on the wrapper caught fire, melted the roof of the oven.
I’ve used spoons and forks in the microwave for years with no problems. If you put a fork in, make sure that the tines of the fork are embedded in the food, so it doesn’t arc between the tines. Ideally, only the handle of the spoon/fork is exposed.
I’ve seen microwave recipes for turkeys, etc. that called for aluminum foil strips strategically placed to control the cooking.
Arjuna34
Newer microwaves are somewhat shielded from the effects of “metal shock.” In fact, unless you own a very cheap microwave, or touch the metal to the actual inside surface of the oven, you’re likely to have no problems. Naturally, don’t try this unless you don’t mind being homeless because your house burned down while you heated your coffee.
BTW, I’ve done the metal thing first hand, by putting an imitation pop tart with a paper cover in the micro, not realizing it had metal inside. Not a nice sound, but the flash was cool, after my heart stopped pounding. Well, what do you expect for $2 an hour babysitting help? 
My son did the exact same thing one time. The only difference was that he said there was major lightning going on inside. The roof also melted. But it still worked fine. 
Metal sparks in a microwave since it acts like an antenna for microwaves. When the waves strike the metal, electrical currents are generated. For many objects, foil included, this flow of electrons is very irregular, so some portions of the object are at a much higher potential (voltage) than others. Create a high enough potential difference, and you get a spark.
My hypothesis is that your spoon was thick and smooth enough to prevent point charges from building up. That’s why the metal walls and metal turntable (in my microwave, at least) don’t spark every time I turn it on.
Of course, the other reason you generally shouldn’t use metal is that it shields microwaves. Hence, Arjuna34’s turkey cooking instructions; you place metal stips over wings and drumsticks to keep them from overcooking.
Well, here’s a few words from Cecil hisself on the subject.
And a mailbag response with yet more interesting experiments to try.
And then of course, there’s grape racing
You first noticed the utensil AFTER you took it out?
A few days previous, I had microwaved a big can’s worth of beans for burrito filling. What I didn’t use on that day I put in that bowl – with the spoon, inadvertently – in the fridge. There were a lot of leftover beans in the bowl, so the spoon sank in without a visible trace.
I mentioned this to my boss, and he told me to watch out because the spoon was now radioactive. He usually doesn’t try to be funny, so I was taken aback.
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If you have never done this, go put a CD in your microwave. For like, 2 seconds. It’s cool. Lots of lightening and the CD looks cool when your done, but obviously unplayable. It will probably smell and any damage done to your microwave I am not responsible for, but I have done this 15 times or so. No microwave has ever blown up.
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deadman
I have to ask. Why?
If I’m going to stop the cooking in the middle to stir the food, I’ll usually keep the spoon/fork stuck in the food, rather than find a place on the counter for a utensil dripping with sauce, or whatever. It’s also slightly faster if the utensil’s already in the food.
Yes, I’m one of those neurotic microwavers who types “99” for one minute and thirty seconds of cooking, because 9-0 seconds is only two keypresses, and 1-3-0 is three, and hitting the same key twice in a row (9-9) is faster than two different keys (9-0). I use 55 or 66 seconds for one minute of cooking
Arjuna34
I am absolutely stunned that your answer makes perfect sense.
Um, thanks, I guess
Arjuna34
Back in my high school days when I was bagging groceries, a co-worker toasted the microwave in our lounge by cooking a loaf of garlic bread complete with foil wrap. The hole in the door was about the size of a watermelon.
I occasionally use the mike to defrost a can of orange juice which still has the metal bottom on. But whay rock the boat? Don’t see the benefit of the energy saved by removing a spoon is worth the risk of blowing an oven.
I don’t think a small, solid object is going to have much affect. The things to watch out for are large surface areas (like a big piece of foil), closely spaced exposed metal (like tines on fork, or intricate foil patterns on china, and metal near the sides of the microwave (specifically, the side where the magnetron is).
A large piece of foil will reflect the microwaves in unpredictable directions, possibly back into the magnetron, or direct them into a tight area. Closely spaced metal will provide spots for arcing.
Anyway, if you have any doubts, don’t put it in- as Dr_Paprika described, a nuked microwave is an easy mistake to make.
Me, I like to live on the wild side- and hey, I didn’t slave away for four years getting an EE degree for nothing! 
Arjuna34
I’ve done it, too. Really, don’t let it go any longer than the lightening show, or the plastic melts and stinks. Also, if you get it before the disk starts to warp, the CD winds up with a very interesting “crazed” appearance while still being flat enough to fit into a player.
Ivory soap is entertaining as well - don’t use a whole bar. It’s similar to what happens with marshmallows.
Light bulbs flicker in a very funny fashion - with a green or purple glow. The filament usually breaks pretty quickly - you can also get a novel effect with neon test lamps or flourescents according to some people, but I haven’t tried it. I’ve often wondered what would happen with old-style photo flashbulbs.
I’ve also tried microwaving ants when they invaded the house. They’re too small - they just crawl around seemingly unconcerned while you’re microwaving them.
All “stupid microwave tricks” come with the warning that I am in no way responsible for any damage that might result should you decide to put any unusual object in your microwave.
My own microwave idiosyncracy is that, except in these rare instances where I’m performing idiotic acts, I will not stand next to the thing while my food cooks. I figure that some of these units are getting old, and if they do start to leak, the inverse square law comes into play.
Cut a grape in half & put it cut side up in the oven for a minute or two. It gives quite a display & there was some scientific debate about this on the net too.
Foil is often used safely to cover areas of food you don’t want cooked.