Q) Do plants have finite and predictable life spans?
Debbie Delacruz, Menlo Park, California

Any grower knows that plants like to die the day before you return from vacation, but it's a different story in the wild. According to Larry Noodén, professor emeritus of botany at the University of Michigan, plant life spans are predictable and generally correlate with reproductive strategy. Monocarpic plants—which form seeds once, then croak—include species that last just a season, plus others like the poorly named century plant, which lives a few decades. Polycarpic plants, which reproduce multiple times, typically survive longer. English ivy endures for 200 years, and bristlecone pines live past 4,000. But the kings of longevity are the asexual reproducers, which keep sending out new roots and shoots. A quaking aspen can do this for 10,000 years. Botanists have yet to uncover any tricks to help Mom's daisies match that, though a study found that feeding them a few milligrams of Viagra keeps stems, um, erect about three weeks longer than normal.

Pop Quiz

A fired-up reader wants to know: why do we use white gas in camp stoves? Though alternative fuels can be used in many modern stoves, none shares all the properties that make white gas uniquely suited for portable cookers. Like gasoline and kerosene, white gas is a distillate of petroleum crude. But its flash point is higher than gasoline's, so an errant spark is less likely to ignite a fireball, and it burns much cleaner than kerosene and thus won't gum up the works. White gas also vaporizes easily, so it can be pressurized via a hand pump, while propane is kept in pressurized canisters that can be dangerous to lug around. Alcohol kicks off less heat, so you'd have to carry more to do the same amount of cooking. And because it's composed of a series of carbon chains, white gas burns well at a range of altitudes and temperatures. All this means there's little to prompt stove makers to hunt for replacements. "We're looking into biodiesel," says Drew Keegan, a product engineer at MSR. "Not because it works great but because it's more environmentally friendly."

Q) So why is the sky blue?
Anonymous, Brooklyn, New York

In short: Rayleigh scattering. The precise explication of this phenomenon involves a complicated mathematical equation formulated in 1871 by Nobel Prize–winning physicist John William Strutt, a.k.a. Lord Rayleigh, with some refinement by Einstein a few decades later. In layman's terms, it goes like this: Visible light travels from the sun to the earth in waves; colors with longer wavelengths, like red, orange, and yellow, pass through the molecules of oxygen and nitrogen that make up the earth's atmosphere, creating the white light that brightens our days. Blue and violet, with much shorter wavelengths, scatter, creating that lovely azure canopy. So why isn't the sky violet, which has the shortest visible wavelength? Actually, to some animals it is—human eyes are just more sensitive to blue. But creatures with better violet perception, like birds and the pop star Prince, live under grape-colored skies.

Q) Is there any health benefit to drinking your own urine?
Eric Lanners, Minneapolis, Minnesota

In the belief that drinking urine promotes wellness, an estimated three million Chinese start their days with shots of home brew. Some New Agers claim a daily dram of wee does everything from cure cancer to increase libido. Most urologists disagree, saying you might develop an infection if your urethra hosts bacteria or viruses. Small doses are usually harmless—urine is 95 percent water and 5 percent waste products like urea, salt, and ammonia—but wilderness-medicine expert Buck Tilton advises against tapping your tinkle even in a survival situation, as salt increases thirst and urea can induce vomiting. A carbon microfilter removes biological baddies and many chemicals but not salt or heavy minerals, notes Shawn Hostetter, sales VP for filter maker Katadyn. For that you need a reverse-osmosis system like the one on the International Space Station, where astronauts quaff recycled whiz. Says Hostetter, "That's probably why they like Tang so much."