One of the hottest technologies in Silicon Valley is also one of the simplest.

The online service from Web start-up Twitter Inc. prompts users to do one thing: answer the question, "What are you doing?" in 140 characters or less.

People type these brief updates, known as "tweets," into Twitter's site or send them to Twitter as text messages. Friends and colleagues can then check the site to monitor each other's updates.

When the service first appeared a couple of years ago, its appeal seemed largely limited to narcissists who wanted to let everybody know what they were doing in real time.

But, like blogs and social-networking sites, Twitter is starting to cross into the mainstream, as a wide range of people find interesting uses for the brief notes.

Doctors are using Twitter to update patients about office hours. Local groups such as the Los Angeles Fire Department are using it to share details about service calls with interested residents, occasionally with graphic descriptions of the victims' conditions.

And dozens of major companies, like computer maker Dell Inc., use Twitter to share deals and product news with people who sign up for the service.

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