15. The system of claim 14 , wherein the operations further comprise: deriving the keyword from the first query in part by dropping the word from the first query.
14. A system, comprising: one or more data processing apparatus; and a memory coupled to the one or more data processing apparatus having instructions stored thereon which, when executed by the data processing apparatus cause the data processing apparatus to perform operations comprising: receiving a first query associated with a user request; determining a score for a word in the first query based at least in part on a first count of activities representing user interaction with content items served for display in response to a past query that included the word and a second count of activities representing user interaction with content items served for display in response to another past query that did not include the word; selecting a keyword derived from the first query based at least in part on the score; and identifying candidate content items using the selected keyword.