21. Non-transitory computer readable media storing software comprising instructions, executable by one or more processing device, that upon such execution cause the processing device 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.
21. Non-transitory computer readable media storing software comprising instructions, executable by one or more processing device, that upon such execution cause the processing device 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. 23. The computer readable media of claim 21 , wherein instructions also cause the processing device to perform operations comprising: selecting one of the candidate content items that matches the keyword; and transmitting information reflecting the one of the candidate content items to a user device.