Q: What do bones contribute to a stock? I can understand how meat and fat impart their flavors, but do the bones break down somehow? Or do we just put them in for the marrow?

A: Dem bones are an important ingredient in beef, veal and chicken stocks. We may think of them as hard and nonreactive, but they aren't made entirely of mineral materials-- calcium phosphates, to be specific. If they were, they certainly wouldn't contribute any more to the stock than a stone would, because calcium phosphates don't dissolve or decompose in hot water.

But bones also contain organic, as opposed to mineral, materials--most notably cartilage (gristle) and collagen. In young animals, the bones can actually contain more cartilage than mineral, and cartilage contains collagen. The fibrous collagen breaks down into soft gelatin when cooked, so bones contribute a rich and unctuous mouth feel to the stock. Shin and thigh bones, together with their connecting knuckle joints, are particularly rich in collagen.

Although bones appear to be solid, they contain a surprising amount of water, nerve fibers, blood vessels and other stuff that would make an instant vegetarian out of you if I told you. In Bones 101 you would learn that a typical bone is made up of three layers. The inner core is a spongy material containing lots of yummy organic matter and, in the hollows of the long bones, the even yummier marrow. That's why--and this is important--we cut or crack open the bones before putting them in the stockpot. Outside the core is the hard, largely mineral layer followed by a tough, fibrous outer membrane called the periosteum.

But the bones we throw into the stockpot have hangers-on, too. Outside of an anatomy lab, have you ever seen a perfectly clean bone, without any meat, fat, gristle or other connective tissue clinging to it? Not likely. All those bits contribute greatly to the flavor. And they brown so beautifully when we roast our veal bones before committing them to the pot when making a dark stock.

Q: Why do they say the meat nearest the bone tastes sweeter?

A: "Sweet" is an overused and misused expression in gastronomic parlance, and is not to be taken literally. But is the near-bone meat really tastier? Often, yes, for several reasons.

First, because it's buried down inside the meat, the bone and its surroundings don't get as hot and cook as fast as the outer parts do. When you grill a T-bone steak, for example, the meat near the bone ends up rarer than the rest and the rarer the meat, the juicier and more flavorful it is.

Another effect arises from the abundance of tendons and other connective tissue that anchor the meat to the bone. As I've said, the protein collagen in these tissues breaks down when heated and turns into gelatin, a much softer protein. But gelatin has the further property of being able to hold huge amounts of water--up to 10 times its own bulk. So in general, wherever there is the most collagen--and that's usually next to the bone--the meat will be both more tender and juicier.

A third meat-near-bone effect is more obvious. In certain cuts, notably in ribs and chops, there is a lot of fat near the bone. So when you're gnawing away at one of those bones, you can't help but get a large dose of fat. And much to our regret, saturated animal fat is delicious.

Q: Cookbooks caution that when using a meat thermometer in a roast, it should never touch the bone. Nowhere have I seen an explanation of why. Does the turkey explode or something?

A: I hate warnings without reasons, don't you? All they do is dispense anxiety without information.

Bone is a worse conductor of heat than meat is. For one thing, bone is porous, and the air cells are heat insulators. Also, bones are relatively dry, and much of the heat transfer through a roast is due to the water in the meat. So when most of the meat has reached a certain temperature, it's likely that the regions surrounding the bones will be cooler. They will make the thermometer read too low and fool you into overcooking your chicken, turkey or rib roast.

While we're at it, haven't you wondered how those plastic, pop-up timers in some turkeys and roasting chickens work?

They're based on the fact that every pure chemical substance has a definite, reproducible melting temperature. (Ice's, for example, is always 32 degrees F.) The timer's manufacturer buries a compressed spring in a little capsule of molten chemical with the proper melting point and lets it cool and solidify, thereby trapping the spring in its compressed form. Then, when the poultry's breast reaches the chemical's melting temperature it melts and releases the spring, which pops the plastic stem up.

It's not a good idea to depend on these gadgets, however, because they measure only one small spot in the breast meat, and usually after it's already too dried out. Better to probe into the thickest part of the thigh with a good, instant-reading thermometer--without touching the bone, of course. When the thigh measures about 165 degrees, the bird is done.

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 food or cooking questions to wolke@pop.pitt.edu.