O.K., Computer, Tell Me What This Smells Like

Our sense of smell is gloriously specific. The mellow aroma of butter and flour rising from warm pie crust, the synthetic bite of fresh paint, the familiar odor of a new car--when we get a whiff of something, we know immediately what it is. But this natural delicacy of perception far exceeds our ability to tell how a given molecule, drawn on a blackboard and considered as an abstraction, will strike our noses. Two substances with completely different chemical shapes might smell almost identical, while two others with similar shapes might smell nothing alike. That's in direct contrast to, say, color vision; by examining the wavelengths of light bouncing off a rose or a child's hat, a scientist can say that a human will see them as red or blue (unless the human happens to be color-blind--though, even then, the shade is predictable).