Amphetamines-most famously methamphetamine-stimulate the nervous system, accelerate the heartbeat, and spark a fast-talking, restless feeling of excitement and energy. The inevitable slang name for such drugs: speed. Anything fast is said to be on speed, a metaphor within a metaphor. The linguistic variants have slid by so rapidly that you can be expected to understand when a car or a music video is said to be meth-paced or methamphetaminic. Athletes have used amphetamines as agents of literal speed (in vain); others have used them as dangerous antidotes to plain boredom. Meanwhile, narcotics offer a rush. And the last socially acceptable mood-altering drug in the puritanical 1990s, with alcohol and nicotine on the wane, turned out to be caffeine. Caffeine, we now know, can bring with it, in sufficient quantity, restlessness, nervousness, excitement, insomnia, flushed face, diuresis, gastrointestinal disturbance, muscle twitching, rambling flow of thought and speech, tachycardia or cardiac arrhythmia, periods of inexhaustibility, psychomotor agitation, and several other of the well-known conditions of our accelerated times. But don't worry-chew another chocolate-covered dark roast bean as you swing by a "Coffee a Go Go" Web site, which croons: "Caffeine, your friend and mine! Near and dear to our hearts, not to mention very tight with our synaptic impulses." These are additives to our engines.