Washington - SINCE it's Valentine's Day, let's dwell for a moment on the profoundly bizarre activity of kissing. Is there a more expressive gesture in the human repertoire?

When parents kiss their children it means one thing, but when they kiss each other it means something entirely different. People will greet a total stranger with a kiss on the cheek, and then use an identical gesture to express their most intimate feelings to a lover. The mob kingpin gives the kiss of death, Catholics give the "kiss of peace," Jews kiss the Torah, nervous flyers kiss the ground, and the enraged sometimes demand that a kiss be applied to their hindquarters. Judas kissed Jesus, Madonna kissed Britney, a gambler kisses the dice for luck. Someone once even kissed a car for 54 hours straight.

Taxonomists of the kiss have long labored to make sense of its many meanings. The Romans distinguished among the friendly oscula, the loving basia and the passionate suavia. The 17th-century polymath Martin von Kempe wrote a thousand-page encyclopedia of kissing that recognized 20 different varieties, including "the kiss bestowed by superiors on inferiors" and "the hypocritical kiss." The German language has words for 30 different kinds of kisses, including nachküssen, which is defined as a kiss "making up for kisses that have been omitted." (The Germans are also said to have coined the inexplicable phrase "A kiss without a beard is like an egg without salt.") How did a single act become a medium for so many messages?

There are two possibilities: Either the kiss is a human universal, one of the constellation of innate traits, including language and laughter, that unites us as a species, or it is an invention, like fire or wearing clothes, an idea so good that it was bound to metastasize across the globe.