India, Dec. 13 -- What does it take to make a room (or car, or cupboard) smell good?

Air-fresheners may seem simple enough: they're a perfume that spreads across a space, via aerosol, slow-release tablet or plug-in diffuser.

They do rely on some clever science to get the job done, though. A particularly tricky aspect, for instance, is controlling the scent-release curve.

At their core, all air-fresheners revolve around volatile organic compounds, tiny scent molecules that are infused into a base and evaporate into the air when released.

How long we can smell a volatile compound depends largely on its chemical structure. Some evaporate in seconds (typically, small, light molecules such as those released by citrus produce). Others last ...