system
Given the following post, respond as if you're commenting on Reddit:
human
Why is putting metal inside a microwave dangerous, but the metal on the inside wall (and other general metal construction) of the microwave not affected?
gpt
The electromagnetic waves in a microwave oven have different effects with varying metal geometry.  The metal walls and metal mesh screen over the door are meant to contain the EM field.  It cannot penetrate sheet metal, even if there are very fine holes.  (If the holes were larger, then the field could escape through them.)

You can technically put some metals in the microwave saftely and see no effect.  For instance, you could put a spoon with a very round handle into a bowl of, say, mashed potatoes, and turn the microwave on and it's possible that it wouldn't damage the microwave or jeopardize your health and safety.

Problems arise when you have pointed objects, or metal objects touching/nearly touching.  The problem with pointed objects is that the microwave induces charges on the surface of the metal.  Where the metal comes to a point, like at the tip of the tines of a fork, the charge density is high, and the fork can arc.  The same thing happens where metal touches or almost touches metal - induced voltage differences will cause arcing.

The spoon in the mashed potatoes can arc if it gets too close to the metal walls, perhaps due to a turn table.  Foil is hazardous as well.

In the end, how the electromagnetic field affects metals in the microwave is dependent on the metals' geometry, and RF engineers have carefully studied the metal walls to make sure they are safe.  While you can get away with certain metal objects in the microwave, it's advised to not attempt putting any metal objects inside, to avoid sparking.

As an aside note, you can also get nonmetallic objects to arc.  One such example is a grape, cut into hemispheres, with a bit of skin connecting the halves.