Olfaction (the sense of smell) is, as Lancaster notes, “a profound mystery”. Why is it, for example, that two aromatic molecules with almost identical structures can smell completely different from each other?
Take this molecule, R-carvone, which smells of spearmint (and also elicits a cooling sensation, because it binds to, and activates the “cold” receptor TRPM8). It is one of two enantiomers, or mirror images, of the carvone molecule.
S-carvone is chemically and physically identical to R-carvone. But, like your left and right hands, the two cannot be superimposed; they rotate plane polarized light by an equal amount, but in opposite directions; and, while R-carvone smells of spearmint, S-carvone smells of caraway.
According to Turin (who was one of my tutors when I studied neuroscience as an undergraduate), these two molecules, and other pairs with similar structures, smell differently because of quantum tunnelling effects in olfactory receptors.