Sandra Stipelkovich, aged 12, of Daly City, California, for her question:

How are mirrors made?

Sometimes a dim old mirror is sent to be resilvered. It is given a new backing and comes home bright as new. The stuff generally used for this renovation job is silver nitrate. This is how the mirror was made into a mirror in the first plane. It was given a backing of silver nitrate, or some similar metal.

Silver nitrate is a salt made by dissolving silver in nitric acid. The liquid is then evaporated and pale crystals of silver nitrate are left. A metallic coating of the stuff is applied evenly to one side of a sheet of fairly heavy glass. The result is a tell­tale mirror. It will show your face as it is ‑ except that it will reflect it in opposites. In the mirror you see the left side as the right side and the right side as the left side,

The trick is done with bouncing light, The dark back of the mirror refuses to let the light pass through it. So the light rays bounce right back. Light rays carry your image towards the glass. The image is reversed as the rays bounce back towards you.