system
Given the following post, respond as if you're commenting on Reddit:
human
What makes some plates, containers, etc. "microwave safe"?
gpt
Microwaves create oscillating electromagnetic fields. These fields will create electrical currents in materials with free charge carriers, ie basically they act as an antenna. They will also cause polar molecules to rotate with the field which leads to heating. Polar molecules are common in food, most notably water, so this is how a microwave heats. The effect is called dielectric heating. If the material is neither conductive nor highly polar, it won't absorb or interact strongly with the microwaves. So what is unreactive to the microwaves? Most ceramics, glasses, plastics, paper. 


If you have a metal, conductive materials due to a lot of free charge carriers, the electrons will move the with field. This leads to multiple undesirable and dangerous effects;

* Small air gaps between metal, like the wrinkles of tin foil or the prongs of a fork, will cause sparking. Sparking will ignite anything flammable. 

* Any metal with current flowing through it is going to heat up. This is non-ideal because you want to heat the food, not the dish. Beyond wasting energy on the wrong thing, heating the dish can again lead to high temperatures igniting something or cause burns to the user when goes to remove it. Some things like microwave popcorn bags intentionally have small metal foil layers to get hot enough at the surface to cook the popcorn properly. 

* Metals will also reflect the microwaves if oriented correctly. You'll note the walls of the microwave are metal. Reflecting the microwaves in unintended ways can damage the microwave by reflecting them back into the magnetron (microwave generator) and overheating it. 

Containers with metal are not always obvious like a stainless steel bowl. Some ceramic dishes contains traces of metals, especially in any glazing or shinny accenting wires they have. This makes some ceramic containers/dishes not microwave safe. Some paper products may also have a metallic foil layer. Don't microwave your tatrapaks for example. If the food is in paper, be careful for any sort of metallic water insulating layer. Wax water insulated paper is fine. 



While most plastics do not strongly react with microwaves, they will be heated in the microwave simply by contact with the food. Some plastics will melt at low temperatures, others will leach chemicals into the food even if they aren't melting. Whether a plastic is marked microwave safe depends on whether it will melt or leach. Most plastics that are microwave safe have a clause stating up to 100C or something, which is hard to avoid locally, especially with dryer food, and after time you will start to notice little white deformities where they slightly melted. Pretty much all plastic will leach slightly, but how much and how toxic it is depends on the plastic. Polycarbonate bottles with BPA were largely banded and discontinued a few years back as they were leaching even at room temperatures. 


So what is safe? Glass always. Ceramics most of the time, though check if they are marked. If they aren't marked, be weary of glazes and shinny strips. Try them out and see if they get hot. Paper most of the time, though if may get flimsy from steam and don't put in any with a shinny metallic foil on it. Plastics (or styrofoams) if they are marked safe, but note they still might leach some stuff and avoid getting them too hot.