15. A system comprising: a first data store storing two or more previous search queries that were previously received from a particular user device; one or more computers operable to interact with the data store; and a second data store storing instructions executable by the one or more computers that upon such execution cause the one or more computers to perform operations comprising: determining that at least two or more previous search queries, previously received from a particular user device, are search queries for a same topic; determining that the particular user device requested presentation of fewer than a specified number of web pages referenced by search results provided to the particular user device in response to the previous search queries; identifying, in response to determining that both the two or more previous search queries are related to the same topic and that fewer than the specified number of web pages referenced by the search results were requested, a related query for the same topic as the two or more previous search queries, the related query being different than each of the previous search queries; receiving a current search query from the particular user device, the current search query being received subsequent to the two or more previous search queries; and providing, for presentation on a search results page for the current query, the identified related query to the particular user device.
15. A system comprising: a first data store storing two or more previous search queries that were previously received from a particular user device; one or more computers operable to interact with the data store; and a second data store storing instructions executable by the one or more computers that upon such execution cause the one or more computers to perform operations comprising: determining that at least two or more previous search queries, previously received from a particular user device, are search queries for a same topic; determining that the particular user device requested presentation of fewer than a specified number of web pages referenced by search results provided to the particular user device in response to the previous search queries; identifying, in response to determining that both the two or more previous search queries are related to the same topic and that fewer than the specified number of web pages referenced by the search results were requested, a related query for the same topic as the two or more previous search queries, the related query being different than each of the previous search queries; receiving a current search query from the particular user device, the current search query being received subsequent to the two or more previous search queries; and providing, for presentation on a search results page for the current query, the identified related query to the particular user device. 17. The system of claim 15 , wherein the instructions further cause the one or more computers to perform operations comprising: identifying, from a data store, a plurality of previous search queries that were received from the particular user device; identifying, from among the previous search queries, a first search query that is related to a first topic; determining that a second search query, from among the previous search queries, was received after the first search query and is related to a second topic that differs from the first topic; and defining, as a beginning of a search session for the second search query, a time after receipt of the first search query and up to a time at which the second search query was received.