Google recently allowed a reporter from The New York Times to spend a day with Mr. Singhal and others in the search-quality team, observing some internal meetings and talking to several top engineers. There were many questions that Google wouldn"t answer. But the engineers still explained more than they ever have before in the news media about how their search system works.

As Google constantly fine-tunes its search engine, one challenge it faces is sheer scale. It is now the most popular Web site in the world, offering its services in 112 languages, indexing tens of billions of Web pages and handling hundreds of millions of queries a day.

"Someone brings a query that is broken to Amit, and he treasures it and cherishes it and tries to figure out how to fix the algorithm," says Matt Cutts, one of Mr. Singhal"s officemates and the head of Google"s efforts to fight Web spam, the term for advertising-filled pages that somehow keep maneuvering to the top of search listings.

Some complaints involve simple flaws that need to be fixed right away. Recently, a search for "French Revolution" returned too many sites about the recent French presidential election campaign — in which candidates opined on various policy revolutions — rather than the ouster of King Louis XVI. A search-engine tweak gave more weight to pages with phrases like "French Revolution" rather than pages that simply had both words.

At other times, complaints highlight more complex problems. In 2005, Bill Brougher, a Google product manager, complained that typing the phrase "teak patio Palo Alto" didn"t return a local store called the Teak Patio.

So Mr. Singhal fired up one of Google"s prized and closely guarded internal programs, called Debug, which shows how its computers evaluate each query and each Web page. He discovered that Theteakpatio.com did not show up because Google"s formulas were not giving enough importance to links from other sites about Palo Alto.

Freshness, which describes how many recently created or changed pages are included in a search result, is at the center of a constant debate in search: Is it better to provide new information or to display pages that have stood the test of time and are more likely to be of higher quality? Until now, Google has preferred pages old enough to attract others to link to them.

Mr. Singhal introduced the freshness problem, explaining that simply changing formulas to display more new pages results in lower-quality searches much of the time. He then unveiled his team"s solution: a mathematical model that tries to determine when users want new information and when they don"t. (And yes, like all Google initiatives, it had a name: QDF, for "query deserves freshness.")

THE QDF solution revolves around determining whether a topic is "hot." If news sites or blog posts are actively writing about a topic, the model figures that it is one for which users are more likely to want current information. The model also examines Google"s own stream of billions of search queries, which Mr. Singhal believes is an even better monitor of global enthusiasm about a particular subject.