Why do metal object in the microwave heat up while the walls stay cool? I just came accross Why is Microwaved mac & cheese burnt where they touch? and discussed it with a colleague. We understand that metal objects heat up due to the eddy currents induced by the absorbed microwave radiation. But then I wondered why the walls of the microwave do not heat up but rather yield the standing waves.
What is different between metal objects inside and the walls that delimit the microwave oven?
 A: 
Why do metal object in the microwave heat up while the walls stay cool?

The objective of microwaves is to be absorbed by the water molecules in the food and cook it.
Have a look here

When it comes to metals though, the wiggling microwaves don’t find any easily excited water droplets, so there’s no good way to turn that wave energy into heat. Instead, two different things happen. A little bit of the wave energy shuffles the loose electrons on the surface of the metal around, and the rest of the wave simply gets reflected. In that way, a sheet of aluminum foil works more like a microwave mirror than a microwave sponge. Metal in a microwave absorbs so little energy that it doesn’t even get warm to the touch.

So it is not that the metal is heated. It is what happens when the electrons in the conduction band of the metal get involved.

First, if there’s nothing but metal in the microwave, you’re left with a lot of mirrors and no absorbers. Pumping more and more energy into the box with nowhere for it to go leads to waves that reinforce each other and build in amplitude. As you run longer, the waves bouncing around grow like a tidal wave approaching shore and eventually get big enough to “cook” the oven’s internal circuits.
Second, if the metal you put in the microwave is pointed (like a fork), has sharp corners (like crumpled aluminum foil), or is thin (like the decorative gold on your fancy teacup), then the electrons moved around by the microwaves can bunch up along the edges. When too much charge builds in one spot, those electrons get very unhappy. And unhappy electrons are liable to—zap!—jump through the air to find a better spot. (That static electric shock you get by shuffling across the carpet and touching a doorknob? Same thing.) While a little bit of sparking between the tines of a fork probably won’t cause long-term damage, larger sparks between the metal and the case are where the trouble starts. Throw a little bit of paper or something flammable in there, and your mother’s fiery disaster prediction will come true.

also

So is it safe to use any metal at all in the microwave? Well, you might not have noticed, but there already is a bunch of metal in your microwave. The walls and circuitry are mostly metal, and the door has a mesh built in that lets visible light out, but reflects microwaves back in .

So in general metal is bad for microwaves as it can start sparks.
The metal walls are in the design and do not get heated because they are built as good reflectors of microwave ( to make the oven work). It goes on to say how to use metal in a microwave .
A: The wavelength of the microwaves is about 12cm, so things like the ends of a tie up on a plastic bag, if they are about 6cm end to end, form a 1/2 wave aerial with the current point in the middle and voltage points on the ends, which arc into the air setting fire to the plastic bag, or, if close to the cavity wall, will arc onto it. The current will probably also blow the thin gold pattern on your crockery like a fuse.
