Patrick McBrearty, age 14, of Huntsville, Alabama, for his question:

Why do they put jewels inside watches?

Jewels are famous for their beauty and the proper place for them should, you would think, be out on display. It seems odd to hide them inside the workings of a watch. Nevertheless, we are told that this watch has five jewels inside it, this one seven or some other uneven number. All of these are fine watches, and the finest may have 23 jewels. These jewels are small, smooth scraps of ruby, diamond or sapphire.

A fine watch is a small miracle of delicate machinery. It has interlocking wheels of assorted sizes, hair thin springs and dozens of miniature parts all fitted together with infinitesimal bolts and screws. Special alloys that expand evenly are used for the metal parts to keep the delicate machinery in balance. But corundum jewels work best as pivots, bearings and other moving parts. These jewels are hard and very durable and normal changes in temperature fail to make them expand or contract. They are more depend able than the best alloys. This is why an expensive watch may have its dainty parts made of precious little jewels instead of metal.