What materials should never be used inside the microwave oven?

Aluminum trays, food cartons with metal handles, metal or metal-trimmed utensils, and metal twist ties should all be avoided as they can cause arcing or fire. Paper bags may cause a fire, plastic foam may melt or contaminate food, and wood will dry out and may split or crack. Always transfer food from these containers into microwave-safe dishes before heating. (Page 10)