5. A computer-readable medium whose contents cause a computing system to perform a method for constructing an index usable to identify expanded queries for a set of seed queries, the method comprising: receiving a query history identifying, for each of a plurality of queries received from users, text of the query, a user submitting the query, and a time at which the query was submitted; for each unique query identified by the query history: for each of a plurality of publicly-available information sources: submitting the unique query as a query to the publicly-available information source; receiving a response to the submitted query; and storing text contained by the response in an entry of the index for the unique query receiving a set of seed queries; for each of the seed queries of the set: identifying an entry of the index corresponding to the seed query; submitting to a search engine a request to match the text stored in the identified entry against the text stored in the entries of the index other than the identified entry; receiving from the search engine a score for each entry of the index other than the identified entry indicating the extent to which the text stored in the identified entry matches the text stored in the entry; and selecting as expanded queries the queries corresponding to the entries of the index other than the identified entry having the highest scores.
5. A computer-readable medium whose contents cause a computing system to perform a method for constructing an index usable to identify expanded queries for a set of seed queries, the method comprising: receiving a query history identifying, for each of a plurality of queries received from users, text of the query, a user submitting the query, and a time at which the query was submitted; for each unique query identified by the query history: for each of a plurality of publicly-available information sources: submitting the unique query as a query to the publicly-available information source; receiving a response to the submitted query; and storing text contained by the response in an entry of the index for the unique query receiving a set of seed queries; for each of the seed queries of the set: identifying an entry of the index corresponding to the seed query; submitting to a search engine a request to match the text stored in the identified entry against the text stored in the entries of the index other than the identified entry; receiving from the search engine a score for each entry of the index other than the identified entry indicating the extent to which the text stored in the identified entry matches the text stored in the entry; and selecting as expanded queries the queries corresponding to the entries of the index other than the identified entry having the highest scores. 8. The computer-readable medium of claim 5 , the method further comprising constructing a segment definition based on the selected expanded queries.