system
Given the following post, respond as if you're commenting on Reddit:
human
Why does certain metal such as metal shelves and the metal lip on microwavable soup cans not cause sparks when microwaved?
gpt
Metals in a microwave only spark when they are pointy. That's because the metal is acting like an antenna, the microwaves inducing an electrical potential in the metal. If the metal has a pointy edge, that electric potential can exceed the dialectric breakdown threshold for air, resulting in sparks (non-pointy objects spread the charge around, which means the voltage at any point is too low to cause arcing.)

Smooth metallic objects are not at risk of this arcing. Forks, in particular, have a tendency to spark, while spoons generally will not. Note that in any case you shouldn't stick metal objects of any kind in unless they are known to be microwave safe, but not all metal objects are hazardous.