What, exactly, do water filters do? I bought a Brita pitcher and it claims to eliminate things like lead and copper with "ion exchange resins," whatever they are. Do they also remove useful things like fluoride?

The name "water filter" is misleading. The word "filtered" means only that the water has passed through a medium containing tiny holes or fine passageways that screen out suspended particles. When you travel in a country whose water supply is suspect and you ask a waiter whether the water is filtered, an affirmative reply may mean little more than that you can see through it.

Here at home, "filter" has become a generic word for devices that do more than clarify the water; they purify it by removing tastes, odors, toxic chemicals and pathogenic microorganisms. The idea is to make the water safer and more palatable. Note that we're talking here about water for drinking and cooking, not for washing. Hard water contains calcium and magnesium, which cause problems in the laundry and in water heaters but are perfectly safe to drink. You'll find water softeners under water in your phone book.

Your nose and palate will tell you whether you want to remove odors and tastes. As far as toxic chemicals and pathogens are concerned, an analysis can be provided by some local water companies or an independent lab. Depending on your degree of paranoia, you may feel like searching for a filter that will remove everything from the water but its wetness. Keep in mind, though, that it's a waste of money to buy a device to remove things that aren't there.

What kinds of "bad stuff" can contaminate water? Industrial and agricultural chemicals, chlorine and its byproducts, metal ions and cysts, which are tiny chlorine-resistant capsules of protozoan parasites such as cryptosporidium and giardia that can cause abdominal cramping, diarrhea and even more serious symptoms in people with weakened immune systems.

Commercial water filters, which may be either batch-at-a-time pitchers or attachments to faucets or supply lines, remove contaminants in three ways: with charcoal, with ion exchange resins and with actual particle filters.

Cryptosporidium and giardia cysts are generally bigger than one micron or 40 millionths of an inch, so any barrier with holes smaller than that will screen them out. But not all filter devices contain such particle filters, so if these contaminants are of concern to you, check the product's literature to see if the performance claims include cyst reduction.

The workhorse of most water filters is activated charcoal, a material that has a prodigious and indiscriminate appetite for chemicals in general and gases (such as chlorine) in particular. Charcoal is made by heating organic matter such as wood in a limited supply of air, so that it decomposes into porous carbon but doesn't actually burn. Depending on how it is manufactured, the charcoal can contain an enormous amount of microscopic internal surface area. An ounce of so-called activated charcoal -- the best kind is made from coconut shells -- can contain some 2,000 square feet of surface area. That surface area makes a highly attractive landing field for wandering molecules of impurities in water or air, and that's where they will land and stick.

Activated charcoal is used to adsorb colored impurities from sugar solutions and to adsorb poisonous gases in gas masks. (That wasn't a misprint. "Adsorption," with a "d," is the sticking of individual molecules to a surface, while "absorption," with a "b," is the wholesale soaking up of a substance. Charcoal adsorbs; a sponge absorbs.) In water filters, the charcoal removes chlorine and other odoriferous gases and a variety of chemicals such as herbicides and pesticides.

Now about those ion exchange resins. They're little plasticlike granules that remove metals such as lead, copper, mercury, zinc and cadmium. These are, of course, not present in the water as chunks of metal, but as ions. ("Uh-oh! Here he goes with the chemistry.")

When a chemical compound of a metal is dissolved in water, the metal goes into the water in the form of ions: positively charged atoms. We can't just pluck these ions out of the water with charcoal, for example, because removing positive charges would leave the water negatively charged, and Nature makes that a very costly operation in terms of expended energy; she vastly prefers the world to remain electrically neutral.

What we can do is exchange those positive ions for other, more harmless positive ions: sodium ions or hydrogen ions, for example. And that's what an ion exchange resin does. It contains loosely bound sodium or hydrogen ions that can swap places with metal ions in the water, leaving the metals effectively trapped on the resin. The resin (as well as the charcoal) eventually becomes fully loaded with contaminants and must be replaced. How long it continues to work depends on how contaminated your water is. If your water is hard, the ion exchange resin will also remove calcium and magnesium ions, and you'll have to replace it sooner.

Most domestic water filters contain both activated charcoal and an ion exchange resin, usually mixed together into a single cartridge. They therefore remove metals and other chemicals, but not necessarily pathogenic cysts. As I've said, check the claims about cysts in the product literature.

Oh, and do the purification filters remove fluoride? No. Fluoride is a negatively charged ion, not a positively charged one. So it is ignored by the ion exchange resin, which has only positive ions to swap.

Labelingo: The labels on Campbell's line of chunky soups say, "Soup that eats like a meal." I'll buy it as soon as I can figure out what to feed it.

(Have you noticed any silly things on food labels? Send your Labelingo contributions, along with your name and town, to Food 101, Food Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or to the e-mail address below.)

Robert L. Wolke (www.professorscience.com) is professor emeritus of chemistry at the University of Pittsburgh and the author, most recently, of "What Einstein Told His Barber: More Scientific Answers to Everyday Questions" (Dell Publications, $11.95). Send your kitchen questions to wolke@pitt.edu.