If you run your microwave with nothing in it the microwave will be damaged and the magnetron will be overheated.
Running your microwave for a few seconds with nothing in it should do no harm but longer than a few seconds could damage the microwave.
You can microwave Styrofoam although you must be careful to not microwave the Styrofoam too long or it will melt.
Microwaving Styrofoam is okay as long as you only microwave the Styrofoam for only 1 to 2 minutes at most.
You can microwave glass bowls and some glass bowls are labeled as being microwave safe.
However if the glass bowl is not labeled as microwave safe you should microwave it for a short time and see if it has any cracks in it.
You can microwave plastic bowls as well as plastic plates and plastic cups and even plastic Tupperware containers.
Even ceramic bowls and ceramic plates can be microwaved as long as they have no metal on it.
You can microwave plastic plates as well as plastic cups or plastic mugs but you must just make sure you don't keep them in the microwave too long or longer than a couple of minutes or they could possibly melt.
You can also microwave paper cups for 1 minute at a time but no longer than 2 minutes or they could burn.
Even Styrofoam cups can be microwaved for 1 to 2 minutes.
You can microwave Styrofoam cups but you should only microwave Styrofoam cups for 1 minute at a time so that they don't possibly melt.
Microwaving Styrofoam cups longer than a minute can result in the Styrofoam cup melting.
You can microwave plastic cups but don't microwave the plastic cup longer than a few minutes at a time or it may melt.
Plastic mugs can be microwaved but they should only be microwaved for 1 minute at a time and a maximum of 2 minutes or they could possibly melt.
You can microwave plastic coffee mugs for at least 1 minute or 2 minutes at most.
Microwave the plastic coffee mug for 1 minute and then if needed microwave the coffee mug another minute until the coffee or drink is reheated enough.
Just be sure the plastic coffee mug has no metal screws or anything on it.
Most plastic coffee mugs however are free of any metal but I've had a few that did have a few screws for the handle.
Ceramic coffee mugs can also be microwaved but stainless steel coffee mugs cannot be microwaved as they will spark and cause a fire or damage to the microwave.