Q. It's the second most heavily traded commodity in the world after petroleum products, with a retail value of $70+ billion and 100+ million people depending on it for their livelihood. Originating in Africa, it eventually spread to the Middle East, to Europe, to South and Central America and beyond. Derived from berry seeds of a family of flowering plants, it has over 100 species (though only two are commonly traded). Its kick comes from the alkaloid 1,3,7-trimethylxanthine and, when roasted, it releases more than 800 detectable volatile compounds in its aroma. Try to identify this popular, decidedly addictive "black wine"? --S. Joe

A. Perhaps you're at your favorite coffeehouse having a cup as you read this. Then you're probably enjoying either "coffea arabica" or "coffea canephora" (also known as robusta), the two most common varieties, says Fernando Vega in American Scientist. The first record of coffee as a beverage dates to about 1450 in Yemen, and that alkaloid-kicker is, of course, caffeine.

Q. You're on a busy city street giving directions to a construction worker, a stranger, who suddenly changes into a different person wearing different-colored clothes. Do you think you would notice the switch? --T. Leary

A. Don't be so sure! In an actual study of this sort, subjects were set up to be giving directions when suddenly two people (confederates of the experimenter) carried a large board past, momentarily hiding the "worker" from view. Then a second worker surreptitiously replaced the first.

Amazingly, most of the subjects were so focused on their task that they failed to notice they were now talking to a different person, says David G. Myers in Psychology: Seventh Edition in Modules. For each of us, our five senses take in an estimated 11 million bits of information every second, yet we consciously process only about 40!

In tests where people heard two different messages simultaneously over a headset, one in each ear, and were asked to repeat the left message as it occurred, they could do this but then could say nothing about the right message-- except the speaker's gender and loudness. The listeners couldn't even state the language of the second message.

"Whatever has our conscious attention pretty much has our undivided attention," Myers notes.

Q. Why is the letter 'W' called a double-U instead of a double-V? It sure looks like a double-V. Who mixed up these two letters in the first place? --H. Spencer

A. Norman scribes in the 11th century introduced the "w" to replace the runic symbol "wynn" from Old English, says David Crystal in The Cambridge Encyclopedia of the English Language. It got the name "double-U" because in Middle English "v" and "u" were interchangeable forms, with scribes writing "uu" for the /w/ sound. This old double-identity is still evident in cognate pairs such as flour/flower and suede/Swede (from askoxford.com).

Oddly, the "w" is the only English letter name with more than one syllable, says Wikipedia.org, giving the Web's ubiquitous nine-syllable initialism "www" the irony of being an abbreviation with three times as many syllables as the unabbreviated form World Wide Web.