Q. When a tree grows, a great mass of trunk, limbs and leaves appears, getting larger each year. The ground around the tree does not subside. In fact, the growing roots often raise the ground. The tree gets water from rain, but surely not enough to justify the mass of the trunk, limbs and leaves. The tree converts nutrients from the soil and air for food but, again, the land around the base does not sink. So where does the mass of a tree come from?

C. Van Iderstine

Davidsonville, Md.

A. A whopping amount of it comes out of thin air, and water makes up nearly all the rest.

But nobody knew that 370 years ago, when Flemish physician Jan Baptista van Helmont took on this question in one of the most famous experiments in the history of biology. He was convinced that water was the main ingredient, and set out to prove it by planting a five-pound willow sapling in exactly 200 pounds of soil.

For five years, Helmont added nothing except rainwater to the container. Then he extracted and weighed the tree, which had gained a healthy 164 pounds. He also weighed the soil, which had lost only two ounces. He concluded that water made up the difference by insinuating itself into the dry matter of the plant.

Not bad for a watched pot. However, we now know that water accounts for much of a tree's mass, but by no means all. The rest comes from carbon dioxide in the air that is converted into carbohydrates through the amazing process of photosynthesis. Although CO2 makes up only 330 parts per million of the atmosphere (and rising), it is the sole source of carbon for plants. And they need a lot: In a typical plant sugar such as fructose, carbon makes up 40 percent of the mass! For a nifty discussion of the whole herbal weight-management business, see Life Processes of Plants by Arthur W. Galston (1994).

When I was a kid growing up in Brooklyn, my friends and I would often engage in this odd behavior. One of us would breathe in and out deeply 10 times. Then he would hold his breath and close his eyes, while a trusted buddy wrapped his arms around the subject's chest from behind and squeezed. Within seconds, the deep breather would become limp and unconscious. He'd be laid down gently on the sidewalk, where he'd wake up in about 10 seconds and say, "Wow!" (or words to that effect). What was going on? When I demonstrated this on my teenage son, it worked like a charm. But my wife (who grew up in Yonkers rather than Brooklyn) was sure I was putting him in mortal danger. Was I?

Sam Kazman

Falls Church

How Come forwarded this to the Post's intrepid Health section correspondent Don Colburn. His report:

This stunt, well known to doctors and other former teenagers, shows what happens when a person's brain, even for a few seconds, doesn't get enough oxygen-rich blood.

"It's a common schoolboy trick," says Robert Wise, professor of medicine and pulmonary and critical care at Johns Hopkins School of Medicine. "We use that phenomenon to teach medical students the interaction of respiration and circulation."

Not that he recommends trying it at home. "I would consider it not prudent," Wise said. "Somewhere between getting a tattoo and glue sniffing."

The brief loss of consciousness is caused by two actions -- hyperventilation followed by the chest squeeze -- that combine to restrict blood flow to the head.

When you breathe deeply and rapidly, blood vessels in your head constrict, reducing blood flow. Squeezing the chest impedes circulation of blood from the rest of the body back to the heart (the so-called venous return). That in turn further reduces the supply of blood reaching the brain.

"And you pass right out," Wise says.

When the passed-out person lies down, normal blood flow is restored and consciousness returns. Lying down quickens the revival because blood flowing from the legs to the heart, and from the heart to the brain, no longer has to work against gravity.

Usually, the brief interruption of circulation to the head is not harmful. But in an older person it could cause a stroke. And regardless of age, if the victim is not allowed to lie down quickly enough, the lack of blood flow to the brain could become dangerous. There have been rare reports of cases where people died after losing consciousness and not being in a position to fall down -- for example, if they passed out in a phone booth or in the crush of a crowd.

Sometimes the moon appears much larger in the sky -- especially when it's near the horizon -- than at other times. Why is this? I was told by a friend that "they" don't really understand why it happens. Is this true, or do "they" have an answer?

Zach Roberts

Gaithersburg

"They" have a definitive answer, but you're not going to like it: It's all in your head. Most folks will swear that a full moon rising on the horizon is about 50 percent to 75 percent larger than when it's directly overhead. Wrong. The moon takes up the same amount of the sky (about .52 degrees of arc as seen by you) no matter where it is.

If our heads were labeled like car mirrors, we'd have to wear warning signs saying "objects in this brain appear larger than they are."

You can prove this to yourself by taking time-lapse photos as the moon rises. Or you can look at it through a tube. Note that the space it occupies inside the tube doesn't change as it climbs higher in the sky. In fact, the easiest way to destroy the "moon illusion" instantly is to curl your hand into a tunnel and look at the rising moon through the hole: It immediately seems to shrink.

As for what screwball mechanism causes this illusion -- which sages have been chewing on for at least 3,000 years -- nobody is sure. There are a dozen different explanations, most involving the way we perceive size in perspective and the way we construe the shape of the night sky.

As the illustration shows, things that appear farther away in perspective usually seem larger to us. (Both black rectangles are exactly the same size.) Moreover, we apparently perceive the night sky as a very shallow arch, much more like the ceiling of a room than a hollow half-sphere. As a result, the moon at zenith seems small to us because we "see" it as if it were closer than it is when it's on the horizon. In fact, the distance is the same.

To get a good sampling of various theories, search the Internet for the phrase "moon illusion" and avoid the lunatics.

How do spiders often spin webs attaching ends to spaces 10 to 20 feet apart?

Philip D. Reed

Berlin, Md.

The answer, my friend, is blowin' in the wind. Suppose you're a spider faced with the task of stringing a horizontal "bridge line," as arachnophiles call the top support strand on that kind of web, across a six- or eight-foot-wide opening in the forest. You've got two choices.

One is the costly pay-as-you-go option: If there is sufficient foliage forming an arch over the gap you need to cross, you can fasten one end of your silk strand to a solid spot and then creep up and across the leafy arc, spinning out silk as you go and being extremely careful that the stuff doesn't stick to anything. Then when you finally reach the other side, you can take up the slack and tighten your line. (Typically, spiders eat the surplus silk, recycling those fantastically strong polymers.)

The other, and vastly more attractive, option is the airborne scheme. Instead of running your eight legs off, you just sit on a leaf or twig and squirt out a very lightweight strand of silk. (Many orb-web spiders can make seven separate kinds of different strength and stickiness.) When the wind catches the silk, it blows it over to the other side of the gap. If it sticks there, then all you have to do is tiptoe across this temporary bridge, simultaneously spinning out a strong line that will become the final top support of your web. Once you're on the other side, you snug up the line and tie it off. Voila. The first stage of your combined game snare and dinette is finished.