How are scratch and sniff cards made?
A scratch and sniff card is a stiff card that, when scratched, releases a fragrance. The fragrance comes from a concentrated oily perfume (a readily vaporized liquid) that does not dissolve in water. Millions of tiny bubbles of the perfume are suspended in a liquid, which is incorporated into a plastic medium. This substance, like ink, can be printed on a card. When the plastic medium dries, the bubbles are trapped inside. Scratching this “ink” causes the bubbles to break open and the odor to escape. Source: The Straight Dope, pp. 246-47.
