Q: Many cookbooks say that one should never wash mushrooms because they soak up water like a sponge and that we should give them only a quick rinse or simply wipe them off. But aren't they grown in manure?

A: Soak up water? Not true. Those books are wrong.

Grown in manure? I'm afraid so.

First, the manure.

The common white or brown button mushrooms in the supermarkets (Agaricus bisporus) are cultivated in beds, or so-called substrate mixtures, that can include anything from hay and crushed corn cobs to chicken manure and used straw bedding from horses' stables.

That knowledge bothered me for many years. Repeatedly warned against waterlogging my mushrooms by giving them a bath, however, I resorted to a soft-bristled mushroom brush that presumably whisked away the nasties from dry mushrooms without bruising them. It didn't do much. I sometimes even peeled my mushrooms, a time-consuming pain in the neck.

But as the hymn "Amazing Grace" would have it, "I once was lost, but now am found, Was blind, but now I see." I know now that the mushroom growers compost their substrate material for 15 to 20 days, which raises its temperature to a sterilizing level. The compost, regardless of its origin, is germ-free before it is "planted" with the mushroom spores.

Nevertheless, I can't help thinking that there is more to manure than germs. So I still clean my mushrooms. And yes, I wash them in water, because they don't absorb more than a tiny bit, as I'll show below. Moreover, I seriously doubt that a water wash removes flavor, as some books claim. After all, even if the mushrooms did soak up water, it would come out in cooking, along with any flavor components it had dissolved.

I was always suspicious of the sponge model of mushroom flesh, because it never appeared to me to be the least bit porous, even under a microscope. When I read Harold McGee's book, "The Curious Cook" (North Point Press, 1990), I was vindicated. An equally suspicious type, McGee weighed a batch of mushrooms, soaked them in water for five minutes-- about 10 times longer than any washing would take-- wiped them off and weighed them again. He found that their weight had increased very little.

I have repeated McGee's experiment with two 12-ounce packages of white Agaricus mushrooms (a total of 40 mushrooms) and a 10-ounce package of brown ones (16 mushrooms). I weighed each batch carefully on a laboratory scale, soaked them in cold water with occasional stirring for McGee's five minutes, threw off most of the water in a salad spinner, rolled them around in a towel and weighed them again.

The white mushrooms, which were all tightly closed buttons, had absorbed only 2.7 percent of their weight in water. That's less than three teaspoons of water per pound of mushrooms, in agreement with McGee's result. The brown mushrooms retained more water: 4.9 percent of their weight or five teaspoons per pound. That's probably because their caps were slightly separated from the stems and water was trapped in the gill spaces, not because their flesh is any more absorbent. Many other irregularly shaped vegetables would mechanically trap small amounts of water. And the timid "quick rinse" recommended for mushrooms by many cookbooks would trap just as much as my five-minute soak did.

So go ahead and wash your mushrooms to your heart's content--at least the common supermarket kind; I haven't tested any of the more exotic varieties. But bear in mind that any brown dirt you see isn't manure; it's probably sterilized peat moss, with which the growers cover the composted substrate and through which the mushrooms actually poke their little heads.

And by the way, if you find your mushrooms giving off a lot of water in the saute pan and steaming instead of browning, it's not because you've washed them. It's because the mushrooms themselves are almost entirely water and you've crowded them so much in the pan that the expelled steam can't escape. Saute them in smaller batches or use a bigger pan.

Q: According to my dad, my grandfather used to go into the woods and collect wild mushrooms, which my grandmother would cook. My dad once asked her how she could tell if the mushrooms were safe to eat. She said she always put a silver dollar in the pan with the mushrooms, and if it didn't turn dark with tarnish the mushrooms were okay. My dad and I are wondering what the scientific basis is behind this method.

A: Stop! I hope I caught you before you put Grandma's reputed wisdom to the test. There is no scientific basis whatsoever to the silver dollar trick. It's pure baloney. I'd call it an old wives' tale, except that women who lived to be old wives probably never believed it.

There is no simple way of distinguishing poisonous mushrooms from safe ones, except by knowing and identifying the species. There are tens of thousands of known species of mushrooms, and many of the poisonous ones look very much like the edible ones. I personally don't have a good visual memory for shapes, so I permit myself to pick only two or three species that have no evil twins. I let the experts (or my favorite restaurants) supply me with the cepes, morels, chanterelles, porcini, shiitake, enoki and oyster mushrooms that have so enlivened American cuisine in recent years.

Incidentally, those ubiquitous portobellos that are on every menu these days are not a separate species; they're common brown Agaricus mushrooms that have been allowed to grow big before harvesting.

Your grandfather did your father a disservice, if I may say so, by letting him believe the silver coin test, because I'm sure he knew better. He simply knew his mushrooms. Your grandmother, having complete faith in her husband (they all had complete faith in their husbands in those days) probably bragged about never finding a bad mushroom in the old man's catch. And of course, the sly old dog let her believe in his infallibility. Even if there were an occasional tummy ache or hallucination, his judgment couldn't be questioned because the silver dollar never turned black, right?

Labelingo: The label on a can of French's Potato Sticks says they're "made from 100 percent real potatoes." Will someone please show me a potato that's less than 100 percent real?

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh and the author of "What Einstein Didn't Know--Scientific Answers to Everyday Questions." Send your kitchen questions to wolke@pop.pitt.edu.