If you don't know it by now, we're here to tell you: Putting metal in a microwave is a huge no-no! Many microwaves are lined with metal, which acts like a mirror, bouncing microwaves around the box until they're absorbed by the food. That's great for heating food, but metal, unlike your leftovers, is a great conductor of electricity; it reflects heat rather than absorbing it, and that's a big problem in a microwave. At best, a little metal in the microwave prevents food from cooking properly. At worst, the resulting heat will create sparks that could cause a fire and render your microwave unusable.
Related: 11 Surprising Uses for Aluminum Foil