14. The system of claim 13 , wherein the instructions cause the one or more data processing apparatus to perform operations further comprising: identifying resources that were requested, by the given user device, through user interactions with search results presented during the search session; identifying, for two or more of the resources, a keyword that is associated with each of at least two of the resources; and identifying the keyword as a related query for the first topic.
15. The system of claim 14 , wherein identifying a related query comprises: determining that the previously submitted search query and the current query are each directed to the first topic; and identifying the keyword as the related query for the previously submitted search query in response to determining that the previously submitted search query and the current search query are each directed to the first topic.