1. A method performed by one or more computers, the method comprising: receiving a search query from a user device; obtaining a plurality of search results for the search query provided by a search engine, wherein each of the search results identifies a respective search result resource; selecting, for each search result resource identified by the search results for the search query, one or more topic queries from a query database that includes previously submitted search queries that have resulted in users selecting search results identifying the search result resource; selecting related questions from a question database that includes previously submitted search queries that have been determined to be in question form, each selected related question being a previously submitted search query that has been determined to be in question form and that matches at least one of the topic queries; and transmitting data identifying the related questions to the user device as part of a response to the search query.