Yael: Whew, Don, what have you been up to? You smell like ladies' perfume today!

Don: I know! I was just thumbing through a magazine at the doctor's office. The      perfume from one of those sample cards got all over my hand. How do those cards      stay so smelly?

Y: 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.

D: Well, I know that 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.

Y: Right, so in order to smell something--for instance, 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.

D: How tiny?

Y: Really 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.

D: So that's what happened. From now on, I'll be careful not to break any bubbles      when I come across those perfumed cards!  

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