Probably the most common tool in the country is the knife. You'd be hard put to find a home anywhere without at least one: That comes to millions and millions of blades, most of them dull.

Over the years, the art of sharpening knives has been lost. Most homeowners don't even know what a sharp knife is, and go through life resigned to blades that crush ripe tomatoes or shread the white meat.

It needn't be that way. It's really quite easy to sharpen a knife. All it takes is a good sharpening tool and a little practice.

What is a good sharpening tool? There are lots of gadgets around, but none is as good as an old-fashioned stone.

I've used a lot of stones, but my favorite is the type called Soft Arkansas, made from a vein of quartz found around Hot Springs; I like it because it works fast yet produces an extremely keen edge. Most quick stones are too coarse.

If you can't find one in your hardware store, order from a source such as Woodcraft Supply, 313 Monvale Avenue, Woburn, Mass. 01801 or Leichtung, 701 Beta Drive, No. 17, Cleveland, Ohio 44143. A fairly large bench stone will cost about $15. That's more than a small pocket model, but big stones are easier to use and worth the money.

There's no trick to using a stone. All you have to do is slide the knife over it at the proper angle, one side of the blade and then the other.

The proper angle is about 20 degrees, and it's important to maintain it without a lot of wobbling.

Place the stone on a nonskid surface or fasten it down, leaving both hands free to hold the knife firmly.

If you have trouble visualizing 20 degrees, fold the corner of a sheet of paper in half. That's 45 degrees. Fold it in half again and you get 22 1/2 degrees, close enough.

Spread a film of sharpening oil or any light oil such as 3-in-1 over the stone. Holding the knife at approximately 20 degrees, stroke it firmly over the stone, cutting edge first. Work just as though you were trying to slice a thin layer off the stone.

Stroke one side of the blade and then the other. After about five strokes on each side, look at it. Running along the length of the blade, at or very close to the edge, will be a thin ribbon of shiny, clean metal. This defines the area you have honed.

If the ribbon extends all the way to the cutting edge, along the entire length of both sides of the blade, stop. If not, try again until it does. The knife will be sharper than new.

To make it even sharper you can strop it. Stroke the blade firmly at about 25 degrees-edge to the rear this time-over ordinary cardboard laid on a flat surface. Make about five strokes on each side. It should be sharp enough to shave the hair off your arm.