SAN FRANCISCO  The first reaction many people have to Twitter is befuddlement. Why would they want to read short messages about what someone ate for breakfast?

It’s a reasonable question. Twitter unleashes the diarist in its 14 million users, who visited its site 99 million times last month to read posts tapped out with cellphones and computers.

Individually, many of those 140-character “tweets” seem inane.

But taken collectively, the stream of messages can turn Twitter into a surprisingly useful tool for solving problems and providing insights into the digital mood. By tapping into the world’s collective brain, researchers of all kinds have found that if they make the effort to dig through the mundane comments, the live conversations offer an early glimpse into public sentiment  and even help them shape it.

Companies like Starbucks, Whole Foods and Dell can see what their customers are thinking as they use a product, and the companies can adapt their marketing accordingly. Last week in Moldova, protesters used Twitter as a rallying tool while outsiders peered at their tweets to help them understand what was happening in that little-known country.