Thank you for being a voice in the noble task of dispelling this myth. While it is true that water strongly absorbs microwave wavelength photons, other things do as well to more or less degree. It is all determined by the absorption cross section of the stuff you stick in the microwave.

I believed this for such a long time until I learnt watching my aunt warm empty plates in the microwave.

I thought it would damage the oven like running it empty would, but now heat them like this all the time.

Ahh, so that's why some cups come out of the microwave with a hot handle while other cups come out with a cold handle!

For practical purposes it’s a useful model.

I have to disagree. Any 'model' that implies that water has some resonant frequency that microwaves are specifically designed for is misleading to the point of uselessness.

If microwave ovens were tuned perfectly for water, then the water near the surface of foods would absorb most of the oven's energy, and not let the center of the food warm up as efficiently... essentially causing the microwave oven to heat food more like a regular oven.

AFAIK, the frequency chosen for use by microwave ovens is a balance between

* A desire for the EM energy to penetrate deeper into the food (per my 2nd paragraph above)

* A desire for the EM energy to not interfere with licensed communications bands, in the event that the oven is "leaky".

Finally: it's not very hard to explain the concept of a dipole moment, and why molecules that have one will twist in the presence of EM radiation. In fact, if you do explain this, quite often the student will come to realize that this is also true of light, which is why (A) a vintage Easy Bake Oven toy could bake a brownie with just an incandescent lightbulb, and (B) will serve to demystify microwaves as being just sub-infrared light.

> A desire for the EM energy to not interfere with licensed communications bands, in the event that the oven is "leaky"

I'm not sure how deep a desire this is, given microwaves operate at 2.45 GHz wideband, which is bang smack in the middle of 802.11n Wi-Fi and Bluetooth...

That frequency was chosen because it's not part of licensed bands, doesn't mean the band is unused. Also, microwaves have been around a lot longer than wifi and Bluetooth.

Then come up with a better model for chefs

Well, I kinda did in my last paragraph. But if you want it more ELI5:

"Some molecule shapes rotate like windmills when light or radio waves (same thing) pass close by them. Water happens to be one of those shapes, but there are others in food too, so it is not just about water. Also, there is nothing special about the frequency of microwaves to make this happen; microwave ovens use that frequency because it is considered a "free for all", and you don't have to ask the government for permission to use it for anything (cooking, bluetooth, or otherwise.) The frequency used for the microwaves does make some sense for use in a microwave oven, because that frequency tends to penetrate deeper into food than visible light does."

That's still a model -- because everything is, the map is not the territory -- but it's close enough that I think most internet pedants won't try to refine it more.

It doesn't explain why butter melts but shortening doesn't. sounds like you'll need a better model

... because shortening isn't butter. Someone else in this thread mentioned that fats have dipole moments, but that doesn't mean that all fats react with the same intensity. For any affected substance: molecular weights, crystalline structures, etc all play a role.

You aren't uniquely "ringing water's bell" with 2.45GHz