Anything metal or any kind of aluminum (including aluminum foil) should never under any circumstances be placed inside of a microwave. Furthermore, any kind of plastic or Styrofoam should never be put into these kinds of ovens either since the radiation will quickly melt these products. Avoid putting a wooden spoon or lined tin containers in microwaves either since these kinds of materials are also ultra-sensitive to the radiation. Always make sure the dishes you use are microwavable, since the radiation can heat these materials to very high levels which could cause burns when you go to remove the containers from the oven.