system
Given the following post, respond as if you're commenting on Reddit:
human
What makes something “microwave safe” and another similar material not safe?

I know some heat up and that makes them unsafe, but why do some materials heat up in a microwave and others don’t?
gpt
Well, metals are an issue as the microwaves induce electrical currents in them. Same idea as an antenna. This means that any metal will get quite hot in a microwave. In addition, if there is any points to the metal the currents can build up high voltages that arc, and can start fires. 



As for ceramics that aren't microwave safe, it has to do with the permittivity of the material. A lot of molecular structures are slightly polar, that is one side of the structure has more density of the electron cloud making it negatively charged. The other end positively charged. This makes a dipole. While this charge is bound together and not free to move as monopole charge carriers (like in metals), it can rotate. As an alternating electric field (microwaves) passes through, the dipole will rotate in response to it. The permittivity of the material, the dielectric, is how much the dipoles response to the external electric field being applied. The permittivity has both a real and imaginary strength, the real value is simply how much it responds but contributes back to the electric field. For example, in capacitors we want the dielectric to magnify the electric field strength, but not lose any energy. In a microwave oven, we care about the imaginary value which is how much of the response takes energy out of the electric field and converts to heat. 


This effect is very similar to refraction with light, actually it's pretty much the same thing other than refraction is in terms of the speed it passed through rather than electric field strength. When light passes through water from air it bends according to the refractive index, which is due to the charge of the water molecules responding to the light's electric field. Part of the repsonce it returns as a slightly offset electric field. This returns the energy to the light while simply slowing it's passage. However, thats not the whole story. There's also an imaginary component to the refractive index too, this component represents the absorbion as light passes through. So the higher the imaginary part of the permittivity, the more the dipoles repaond to the electric field and steal energy from it. Some thing like glass isn't polar and as such has very little response to microwaves. It's permittivity is low, both real and imaginary parts. Some ceramics or glazes are polar, so they heat up a lot. A lot of molecules in food are polar, notably water, and will heat up. 


Lastly, plastics. They themselves really don't absorb microwaves. However, they melt easily. If the temperature of the food rises high enough, the plastic will melt making it unsafe.