1. A method performed by at least one processing unit, the method comprising: identifying a seed lexicon term from a plurality of lexicon terms that are identified by a lexicon as members of a semantic class; identifying result documents retrieved from a search engine by queries submitted to the search engine that include the seed lexicon term; extracting a keyword that is shared by the result documents retrieved from the search engine by the queries that include the seed lexicon term; and using the keyword shared by the result documents to expand the lexicon by: identifying, in a query log, specific other result documents that also include the keyword shared by the result documents, wherein the query log does not indicate that the specific other result documents were retrieved from the search engine using the lexicon terms identified by the lexicon as members of the semantic class; identifying another member of the semantic class in a particular query that has been submitted to the search engine to retrieve the specific other result documents; and adding to the lexicon, as a new lexicon term, the another member of the semantic class.