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Here at Collected Wisdom we're ever vigilant about security, which is why we have our own food taster. He's now sampling a lovely cake sent in by our grateful readers. Oh, dear. Should he really be turning that shade of purple?

THE QUESTION

What is the most poisonous substance in the world? asks Jackie Phillips of Toronto.

THE ANSWER

The toxicity of poisons, toxins and venoms is measured by the median lethal dose of the substance needed to kill 50 per cent of those exposed to it, writes Roger Kingsley of Winnipeg. It's known as the LD50, as in lethal dose, 50 per cent.

The LD50 statistic is usually expressed in measures such as the number of grams of the poisonous substance per kilogram of a person' s body weight, he says.

"Every substance is toxic if ingested in large enough quantities. For instance, water has an LD50 of 90." This means that, if a large number of people each weighing 100 kilos (220 pounds) each drank 9,000 grams (20 pounds) of water at one sitting, it would kill half of them.

"At the extreme-toxicity end of the scale," he says, "botulinum toxin has an LD50 estimated at about 0.000000001," or one-billionth of a gram per kilogram. "This is the lowest LD50 generally recorded for any substance."

Botulinum toxin, by the way, is a neurotoxin produced by the bacterium Clostridium botulinum that can cause the severe food poisoning known as botulism.

Intrigued by all this, Collected Wisdom looked up the LD50s of some other lethal and not-so-lethal substances (all in grams per kilogram).

Table salt, for instance, has an LD50 of 3.0, caffeine is 0.19, nicotine is 0.05, cyanide is 0.0064 and strychnine is 0.001.

THE QUESTION

Tom Jarvis of Whistler, B.C., notes that many commercial airliners have wingtips that are turned up at the ends. Why?

THE ANSWER

An aircraft wing produces lift because the air pressure on the top of the wing is lower than the air pressure on the bottom of the wing, writes Gordon Cullen of Vancouver, who flies the Airbus 320 for Air Canada.

"One consequence of this is that some of the higher-pressure air below the wing moves outward along the wing and escapes over the wing tips to the lower-pressure area on top of the wing," he says.

This creates turbulent, spiralling motions in the air known as "wing-tip vortices," which increase aerodynamic drag.

As a result, he says, many modern aircraft have wing tips that are turned up at the ends, and these "winglets" reduce drag and thus save fuel.

Indeed they do, says Peter Cranston of Victoria. "I flew the first Bombardier Challengers. When they were refitted with winglets, we could fly 10 per cent farther on the same fuel load. I always wondered why it took so long to invent them."

Marilyn Baker of Richmond, B.C., can throw some light on how winglets came about.

"A retired aeronautical engineer from Boeing told me that during the design of the 747, they studied the flight of the buzzard," she writes.

"They noticed that the bird has long feathers on its wing tips that turn up, so they incorporated that feature into their design."

HELP WANTED

Pierre G. Gagnon of Oakville, Ont., wonders how many bugs there are (approximately) for every person on Earth.

How do birds find worms that are beneath the ground's surface? David Shore of Richmond, B.C., wants to know.

Eli Goldman of Ottawa asks: Can a pathological liar pass a lie-detector test?

Let's hear from you: If you have the answer to one of these questions (or a question of your own) send an e-mail to wisdom@globeandmail.com. Please include your location and a daytime phone number.

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