19. A system comprising: one or more data processing apparatus; and a computer-readable storage device having stored thereon instructions that, when executed by the one or more data processing apparatus, cause the one or more data processing apparatus to perform operations comprising: obtaining a plurality of first search results responsive to a first search query, the plurality of first search results being ranked in an order; generating a second search query by adding a term to the first search query, the second search query being different from the first search query; obtaining a plurality of second search results responsive to the second search query; determining that a particular first search result responsive to the first search query appears within a threshold number of highest-ranked second search results responsive to the second search query; determining that the particular first search result in the first search results responsive to the first search query is not within a threshold number of highest-ranked first search results of the first search results responsive to the first search query; in response to determining that the particular first search result responsive to the first search query appears within a threshold number of highest-ranked second search results responsive to the second search query and that the particular first search result in the first search results responsive to the first search query is not within a threshold number of highest-ranked first search results of the first search results responsive to the first search query, modifying the order of the plurality of first search results by moving the particular first search result within the plurality of first search results from an original position in the order to a second, different position in the order, wherein the second position is higher than the original; and providing the modified plurality of first search results in response to the first search query.
19. A system comprising: one or more data processing apparatus; and a computer-readable storage device having stored thereon instructions that, when executed by the one or more data processing apparatus, cause the one or more data processing apparatus to perform operations comprising: obtaining a plurality of first search results responsive to a first search query, the plurality of first search results being ranked in an order; generating a second search query by adding a term to the first search query, the second search query being different from the first search query; obtaining a plurality of second search results responsive to the second search query; determining that a particular first search result responsive to the first search query appears within a threshold number of highest-ranked second search results responsive to the second search query; determining that the particular first search result in the first search results responsive to the first search query is not within a threshold number of highest-ranked first search results of the first search results responsive to the first search query; in response to determining that the particular first search result responsive to the first search query appears within a threshold number of highest-ranked second search results responsive to the second search query and that the particular first search result in the first search results responsive to the first search query is not within a threshold number of highest-ranked first search results of the first search results responsive to the first search query, modifying the order of the plurality of first search results by moving the particular first search result within the plurality of first search results from an original position in the order to a second, different position in the order, wherein the second position is higher than the original; and providing the modified plurality of first search results in response to the first search query. 20. The system of claim 19 , wherein the second search query includes a synonym of a term in the first search query.