Phlogiston.
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We produce phlogiston. Phlogiston has thousands of industrial uses, but if you're not involved in manufacturing, you probably don't want to know about those.
For the consumer, you probably want to know that phlogiston is the magical substance that makes mood rings function.
Because of the unique properties of phlogiston in detecting the emotional reactions of people, phlogiston is very useful in espionage. Phlogiston is also indispensable in the high tech industry in all products which are designed to detect when the user is desperate or in a hurry in order to malfunction at the worst possible time.