How are these utensils microwave-safe? These types of utensils are starting to become popular nowadays.




I checked their product description which says that its outer surface ( i.e. the red part) is microwave-safe plastic while the inner surface is made of stainless steel. 
I don't know how this makes the utensils microwave-safe as the microwaves can still interact with the inner surface (closing the lid is not required 
as per the instruction manual). 
I even used one of these and they haven't seemed to cause any problems. I searched a lot on the net about how it is safe but was in vain. 
So, how are they safe?
 A: My first idea is that it's because the metallic surface is smooth. I've heard a few times in my life that you can actually put metallic things in microwave (e.g. aluminium foil) but only as long as they don't have any sharp edges. If the metallic object is pointy or has sharp edges, electrons will collect there and this can cause sparks. A flat metal will just reflect microwaves (which now makes me wonder if the heating of your food will be less effective in a pure metal bowl compared to heating in a non-metallic bowl).
A: Well, your microwave oven is built of metal. The metal of the bowl does not act in any way differently than the metal that the casing of your microwave oven is built of: It simply reflects any microwaves that hit it.
That said, the electrical fields in a microwave oven are quite strong. So, if you put anything with sharp corners/edges in, you may see sparks at those corners. This in, and off it itself is not really dangerous to your oven. It may be dangerous to whatever is near those corners, though.
What is dangerous to your oven, is when the energy of the microwave field has nowhere to go. This happens when you wrap your food into metal. Or put it into a metal pot with a metal lid. In this case the metal stops the microwaves from reaching your food, so the strength of the microwave fields builds up until it damages the oven itself.
Of course, a metal that is a bad conductor will not fully reflect the microwaves, but rather turn some of them into heat. Such metal parts have the potential of becoming very hot when you put them into a microwave.

TL;DR:
Because metals interact so strongly with microwaves, it is generally best to keep them out of the oven. However, parts that were carefully designed to be used with the oven are safe because those effects have been considered while designing them. The difference is that of knowledge between you and the designer of the bowl.
A: There are already some good answers here. Just an addition (https://engineering.mit.edu/engage/ask-an-engineer/why-cant-we-put-metal-objects-in-a-microwave/): "In some situations, metal placed inside a microwave can become very hot, a fact food manufacturers cleverly take advantage of... Some microwavable soups and pies are packaged with a thin metallic layer under a non-metallic lid, so the food trapped against the metal browns nicely. But leave those snacks in just a few minutes too long, and they might incinerate."
