Bathroom glazing—windows, shower doors, and even mirrors—must fulfill safety criteria under federal, state, and municipal construction laws. To put it another way, the glass used in bathroom mirrors must be tempered glass, which breaks into little spherical pieces to help avoid injury. Standard mirror glass is too weak to be safe. Tempered glass can be found in many common objects like your kitchen sink or television. When glass is heated to a high temperature, its atoms become disordered and cannot return to their original position when cooled down. This makes the glass more brittle but also significantly reduces its weight compared to ordinary clear glass of similar size. As a rule of thumb, if you can write with a marker on the glass surface, it's not strong enough to be safe for use as bathroom glass. Mirrors are manufactured from flat sheets of polished metal or plastic that are bent by hand or machine to create the reflective surface. The most common type of metal used in modern mirrors is aluminum, although silver and gold also can be used instead. Plastic mirrors usually are made from polyvinyl chloride (PVC) or acrylonitrile butadiene styrene (ABS). Both metals and plastics are rigid when cold and hard to the touch, but when heat is applied they will begin to soften.