By Patrick Clinton

Why can't we get syrup from trees other than maples? Is oaken pancake syrup possible?
Nick Galbreath, Boston, Massachusetts

You can make syrup out of sap from some other trees, such as the butternut and the box elder, a cousin of the maple. There's even minor commercial production of birch syrup. But for the most part, only the sugar maple has sweet-enough sap to make its harvest worthwhile. You'd have to boil down 150 gallons of birch sap for a gallon of syrup; the ratio is about 40 to one for maple sap.

Sweetness, however, is just part of the story. Most trees don't "bleed" sap like maples do. (Even maples bleed significantly only in late winter when they're preparing to emerge from dormancy.) Oaks have a more evolved circulatory system. They transport fluids only in a narrow ring under the bark, so they heal quickly when cut.

Even if you could collect a bucket of oak squeezins, you might not want them center stage on your flapjacks. According to U.S. Forest Service plant pathologist Phil Wargo, "Ring-porous trees like oaks convert carbohydrates into brown grungy things." The grunge helps fight off fungi, which are big killers of trees. Very useful for the oak, of course, but no perk at the pancake house.

Earth's core is constantly losing heat, right? Should we be worried about global cooling?
Karen Erlandson, Goleta, California

Actually, earth is far from becoming the third snowball from the sun. The core got hot about 4.5 billion years ago, when Earth collided with another planet-size body. The core temperature was 10,000 or 12,000 degrees Fahrenheit then, and it's dropped by only about a thousand degrees since. At that rate, the core will be toasty for tens of billions of years. Besides, it doesn't contribute much to the total planetary energy picture: Scientists measure the heat energy radiated at Earth's surface in milliwatts per square meter, and a football-field-size parcel gives off about 200 wattsenough to power a couple of light bulbs.

Earth's radioactive elements produce new heat all the time. But the main reason the core has stayed hot, according to University of California Berkeley geophysicist Raymond Jeanloz, is that rock is a terrific insulator. "Even if you could instantly turn off the core heat, we wouldn't know it for millions of years," he says. "We'd have volcanoes and earthquakes for a long time to come." So don't you worry.

How do boomerangs work?
Chris Mills, Seattle, Washington

Before I explain this, Chris, I want you to meditate on one of life's mysteries: If you watch the wheel of a bicycle someone's riding, the spokes above the axle move away from you faster than the spokes below the axle. Got it? Take your time.

The wings of a boomerang are like the spokes of a wheel. You throw a 'rang vertically, like a knife, as opposed to horizontally, like a Frisbee, and the wing on top always has more airspeed than the one on the bottom. The wings are airfoils, which means they create lift; the more airspeed, the more lift. (Remember, the boomerang is soaring sideways, so it "lifts" to the side, not up.) If you're still with me, the top wing has more lift than the bottom wing, so lift forces threaten to tip the boomerang over.

But it doesn't tip, because of gyroscopic precession, the same force that makes tops wobble weirdly when you play with them. Precession relocates forces that act on spinning bodies. In this case, it takes the tipping force and moves it 90 degrees forward, so it becomes a steering force. The boomerang turns and keeps turning until it comes back. Simple and elegant. Kind of makes you wish aborigines had designed Windows 95.

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