I had a microwave with a manual, in which it read that to heat liquids faster, you should put a metal spoon in the glass... This really irritated me because I thought it was dangerous to put metal in a microwave... Turns out the body of the microwave is made out of metal itself and its no problem to put metal in the microwave. Depending on the form of the objects, you might get some sparks or Aluminum is so thin it might melt. Otherwise no big deal... Dry foods for example can catch fire, which is way more dangerous.