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Metal objects in microwave ovens are known to be somewhat dangerous.

However, my microwave has the following label stuck inside:

enter image description here

It seems to imply that this particular oven forbids heating drinks without a spoon. And I have been following this advice for years now, without ever getting any sparkle or explosion, so even if this pictogram was meant to be something else entirely, this oven at least has no problem with metal spoon.

Unfortunately, all I have is the label and the name of the oven. It is quite old now, and the manual is long lost. All I could find on the manufacturer's website is this page, which basically confirms that it is a microwave oven (it also contains lists of spare parts and exploded diagrams, which I guess could provide clues to an expert...).

Any idea howmy oven might work? And why would the sticker be so... imperative?

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    $\begingroup$ THis is off topic I think, but microwaves can super-heat water, especially in glass containers, and especially when someone puts the water through multiple heating cycles. If this occurs, the water is past boiling but is not "boiling" visually. When someone then disturbs the container or puts a spoon in to stir it, the water spontaneously boils almost explosively, spraying super-heated water. The diagram is suggesting a spoon so there are nucleation sites where bubbles can form. see Mythbusters or snopes.com/science/microwave.asp for examples $\endgroup$ – Yorik Feb 5 '16 at 22:15
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This sticker is indicating caution when you heat a liquid. A microwave can induce superheated water which can be dangerous. If your water is superheated, it may not explode/boil over until an object is placed inside the water (e.g. a spoon). One way around this is to place a chop stick or object into the water while it is heating. This will prevent that nasty surprise when you make your earl grey, hot.

Now, putting metal into a microwave does no necessarily cause sparks. In fact, you can actually put tin foil in a microwave at times and not create sparks (Disclaimer!! Do not try this at home). The sparks are caused when you have very thin pieces of metal. So when tin foil does spark, it is at the very edges where tearing has caused the material to be thinner than the bulk.

Other fun things to do with a (preferably someone elses...) microwave: this and this.

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    $\begingroup$ Ok, so basically there are two sides of the answer: 1. spoons in a microwave are not actually dangerous ; and 2. superheated water (however cool!) is dangerous and can happen by accident. And it is not specific to my microwave, even though most manufacturers don't feel the need for this warning. I guess I'll get an opportunity to mess with some friends' nerves at times... Thanks! $\endgroup$ – T. Verron Feb 5 '16 at 22:57
  • $\begingroup$ Well I'm not sure I would recommend putting spoons in microwaves still. If there is a gold or silver plating it could still spark $\endgroup$ – Greg Petersen Feb 5 '16 at 23:43
  • $\begingroup$ Hehe, yes, don't worry. On second thought, the manufacturer has got some nerve putting that warning, that's basically trading one risk for another. Even if the manual explains thoroughly that wooden spoons are preferrable, users can't be trusted to read it... $\endgroup$ – T. Verron Feb 6 '16 at 8:47
  • $\begingroup$ Why in the world is there a disclaimer if you say tin foil won't cause sparks... $\endgroup$ – Mehrdad Sep 13 '17 at 23:18
  • $\begingroup$ I said "at times". Tin foil can spark if the edges are thin enough after tearing. $\endgroup$ – Greg Petersen Sep 14 '17 at 18:45
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From the Bosch Microwave/Speed Oven manual:

"Liquids, such as water, coffee, or tea could be overheated. They can be heated beyond boiling point without appearing to boil. For example, visible bubbling or boiling when the container is rmoved from the microwave oven is not always present. THIS COULD RESULT IN VERY HOT LIQUIDS SUDDENLY BOILING OVER WHEN THE CONTAINER IS DISTURBED OR A UTENSIL IS INSERTED INTO THE LIQUID. When heating liquids, always place a glass rod/spoon in the container. This will prevent delayed boiling.

CAUTION Metal e.g. a spoon in a glass must be kept at least 1 inch (25mm) from the oven walls and the inside of the door. Sparks could irreparably damage the glass on the inside of the door."

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