You have most likely encountered the smelly perfume ads in magazines before. Just how do those ads stay so potent?

It's the same technology used to make scratch-and-sniff stickers. The process is called "microencapsulation." To understand how it works, you need to know a little about our sense of smell.

Molecules released by any aroma-producing substance diffuse up into our noses and bind to special receptor cells. Then those cells send signals to the brain that we recognize as odors. In order to smell something, like say a ripe peach, there must be molecules, called odorants, released from the surface of the peach into the air that we sniff. Microencapsulation works by encasing the odorants in tiny plastic or gelatin capsules.

The capsules are tiny. A scratch-and-sniff sticker might have fifty million capsules per square inch. The micro-capsules are produced by whipping an odorant into a solution of liquid plastic. This emulsification process creates billions of tiny balls of odorant, each surrounded by a thin plastic membrane. The bubbles of perfume are then printed onto cards or stickers. The liquid inside is trapped until the capsules are broken by scratching or rubbing, which releases the aroma.