Q. I know that you can siphon water uphill as long as the eventual destination is lower than the beginning. Is there any way to get water uphill without a pump? I'd love to get my wash water from the basement to my drought-stricken garden.

Carol Jo Roeder

Columbia

A. We feel compelled to warn you that your plan would violate several of the more important laws of nature, and Horizon would have to make a citizen's arrest.

The way this particular universe is built, things just naturally tend toward a condition of minimal energy, which is why it's so hard to get kids to take out the trash. The reason that water flows downhill is to minimize its gravitational potential energy -- that is, the energy content that it has by virtue of being at some distance from Earth's center.

The higher the water or anything massive, the more potential energy it has. That's why it's harder to walk up a flight of stairs than down, even though you're moving the same body mass the same distance either way.

Nature is ridiculously fussy about balancing its energy accounts, and the law of conservation of energy obstinately insists that exactly the same total amount of energy must be present before and after something happens. Hoisting your basement water to petunia level would add to its potential energy. So some energy would have to be supplied to compensate for that. A siphon can't do that.

A siphon works precisely because it permits a liquid to minimize its potential energy. As a result, no matter how far the tube may arc temporarily above the source, the fluid level in the receiving container has to be lower than the level at the source for the siphon to work.

The fluid moves because its natural motion toward a lower-energy condition creates an area of low pressure inside the tube; the higher pressure in the source pushes fluid up into the tube. But as soon as the level in the receiving container rises to equal that at the source, the flow no longer goes.

Bottom line: Consider washing your clothes in the garden.

Why do we get sleepy after eating a sizable meal? Some have suggested that it's related to the impact of insulin during the initial stages of digestion. Others have claimed that it is simply blood rushing to the stomach and intestines, reducing blood elsewhere and thereby temporarily causing loss of energy. Which is it, if either?

James M. Talens

District

We woke up Don Colburn, The Post's health correspondent, to handle this question. His report:

There aren't a lot of definitive medical studies of after-dinner zonking out, experts warn. For one thing, it's hard to measure whether people become sleepy after a big meal and whether they think they're getting sleepier than they otherwise would.

Yet common sense and personal experience tell us that it happens.

"There's no single explanation," says Cynthia Reeser, a nutritionist in the Lipid Research Clinic at George Washington University Medical Center.

One factor is indeed the increased blood flow to the stomach and intestines, which reduces the supply available to the rest of the body. Blood goes where it is needed -- for instance, to the leg muscles during a run and to the abdomen during digestion.

"There's an energy cost to the digestive process, and the body has to slow down to accommodate that," Reeser says. (Pig-out portions obviously exaggerate this.)

Another possible factor is the meal's fat content. Fat is relatively difficult to digest. "If the meal is high in fat, you'd feel sluggish longer," she says.

Insulin, a hormone secreted by the pancreas, helps to regulate blood sugar levels but doesn't make people sleepy after a big meal. A brain chemical called serotonin has more to do with that.

Meals containing a high proportion of carbohydrates -- pasta, for example -- stimulate release of serotonin. One of serotonin's many effects is to increase feelings of calm and even drowsiness.

Carbohydrates also are the body's preferred source of energy; athletes "carbo-load" by adding carbohydrates to their diet over a training period. But in a single big meal, a high ratio of carbohydrates to protein can encourage sleepiness.

"Not the kind of meal," Reeser says, "that you want to have at your power lunch."

Soldiers marching across a suspended bridge are advised to go out of step. Why is this necessary?

M. Mital

Fairfax

Because it's better to be safe than soggy. Even the sturdiest structures can be shaken into rubbish by vibrations with the right (or wrong, depending on how you look at it) frequency.

You've probably seen old TV commercials in which a soprano breaks a wine glass by hitting a certain note with her voice. That note is one of the "resonant" frequencies of the glass -- that is, the sound waves vibrate at just the right rate to induce a corresponding vibration in that particular glass and then reinforce the motion until the glass shatters. Other glasses would resonate at other frequencies.

In general, when a wave doing the vibrating matches the "natural" resonant frequency of an object, the object can really start shaking. The problem is that you often don't know in advance what that frequency is going to be.

You may have noticed that something in the dashboard or engine compartment of your car will start to rattle exactly when you hit, say, 42 mph yet won't make a peep at 40 or 44. That's a case of resonant frequency again. But could you have figured out when it would rattle before you drove the car? Not a chance.

Even engineers can be surprised. In the 1930s, folks who designed the Tacoma Narrows Bridge in Washington state did a pretty good job. But they never planned on the bridge being hit by wind gusts of exactly the right frequency to set off resonant vibrations. When that happened one day in 1940, the thing tore itself to pieces.

So when troops march over a bridge, there's a risk that the rhythm of their synchronized footfalls will destroy the structure. But if they stomp out of step, they'll be in synch with physics.

In the midst of the drought, I was wondering: Will Earth ever run out of enough water for us to use?

Jackie Lu

Sterling

For millions of fellow Earthlings, it already has. True, a hasty gawk at a world map is reassuring: Water covers 70 percent of the globe. But human beings can't drink sea water, irrigate crops with it, give it to domesticated animals or use it for most industrial purposes. All of those require fresh water, which makes up only a tiny 2.7 percent of the H2O on the planet.

Nearly all of that -- 2.1 percent -- is trapped in glaciers. In fact, fully 70 percent of all fresh water between here and the moon is ice in Antarctica. That leaves 0.6 percent of the world's water available to us in lakes, rivers, soils, those plastic sucker bottles everybody carries and -- chiefly -- in underground reservoirs called aquifers.

That might still be enough, except that each year it must go a little further.

World population, now 6 billion, increases every 12 months by 80 million or so, roughly the entire population of Mexico. Meanwhile, worldwide irrigation has doubled in the last 40 years, from 343 million acres in 1961 to nearly 700 million now. The amount of available fresh water, however, hasn't doubled.

So by 2025, according to the International Water Management Institute, about 1 billion people will live in countries facing severe shortages. Drink up.

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