Apparently this question was prompted by a remark a guest made on the Dr. Oz Show. While speaking of “toxins” released from plastic he said something about stainless steel being preferable. This caused quite a commotion in chat rooms with some people commenting that only someone with the IQ of plankton would suggest the use of metal in a microwave. I'm not sure how the IQ of Oz's guests compares with that of greenery, but using metal in the micro is far from the silliest thing ever heard on that show.
While metal containers are not appropriate for the microwave, the oven will not catch fire or blow up, as some have claimed. If a smooth metal bowl is used, the only observation will be that the food doesn't warm up. The microwaves will not penetrate the metal; they can, however, induce an electric current in the bowl which is likely to have no consequence unless the metal has jagged edges or points. Then "arcing" can occur and sparks will fly. If there is something combustible in the oven, a fire is possible. The best material for microwaving is glass, although polyethylene or polypropylene are fine. These plastics are marked as "microwave safe."
And while it may be convenient, old margarine or yogurt containers are not microwave-appropriate.
