32. The system of claim 28 , wherein the one of the candidate content items is a search result snippet.
28. A system, comprising: means for receiving, at one or more processor, a first query associated with a user request; means for 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 includes the word and a second count of activities representing user interaction with content items served for display in response to another past query that does not include the word, wherein the means for determining the score includes a processor; means for selecting a keyword derived from the first query based at least in part on the score, wherein the means for selecting the keyword includes a processor; and means for identifying candidate content items using the selected keyword, wherein the means for identifying the content items includes a processor.