Kenneth 3. Mercurio, age 13, of Utica, New York, for his question:

Will you please explain the metric system?

Legislators have been promising to switch us to the metric system for a long time. Every now and then, someone takes a step to bring that sensible day closer. We are happy to report that Andy's sensible penpals are determined to be ready for it. The topic is near the top on our poll of popular questions. The best way to master something like the metric system is to keep tackling it from all angles    so here we go again.

Actually, the metric system is a lot easier to master than the clumsy old measuring systems we use every day. In fact, this is why it was invented    and so far nobody has found anything simpler to replace it. It is based on the decimal system of ten which, let's face it, is as easy as counting ten fingers or, if you prefer, ten toes.

Naturally every measuring system must start with a basic unit. The metric system happens to be based on the French meter, because the whole idea was invented in France. This unit of long measure equals about 39.37 of our ancient inches, which makes it somewhat longer than one of our ancient yards.

The meter is superior because the streamlined decimal system is used to divide it into smaller sections. Goodbye to 12 inches in a foot, three feet in a yard and all those headaches. In the meter there are ten decimeters and in each decimeter there are ten centimeters and each centimeter has ten millimeters. This neat arrangement gives each meter 100 centimeters    or 1,000 millimeters. And what could be neater than 1,000 meters per kilometer, which equals a little more than half of one of our weary old miles.

The rest follows just as smoothly, because the basic meter is used to build the measurements for volume and weight. The cubic meter, naturally, is a box shape measuring one meter on each side. It holds 1,000 cubic decimeters    or 1,000,000 cubic centimeters. The liter, used for measuring units of capacity, is figured from the amount of material contained in a cubic decimeter. It is somewhat less than one of our old style quarts.

The gram is taken from the amount of specially prepared pure water in one cubic centimeter. This is the basis for weight measurement. Two aspirins weigh about one gram and an ounce equals about 2E grams. Ten prams make a decagram and ten decagrams make a hectogram. Ten hectograms make a kilogram, which replaces about two and a quarter pounds.

There are a few other niceties in the metric system, but this explanation gives you the basic idea. In measuring drugs, we already use milligrams and grams, which have 1,000 milligrams. Scientists already use the basic metric system, with extras of their own. The micron, used to measure skinny items such as wavelengths, is 1.000th part of a millimeter. The millimicron is a million times skinnier than the micron. The whole system is put together or separated with nice neat building blocks of ten.