14. A system comprising one or more computers configured to perform operations comprising: receiving a query; receiving a plurality of search results responsive to the query, each search result identifying a resource responsive to the query; after receiving the plurality of search results responsive to the query, accessing an index of resources to determine, for a plurality of the identified resources, that each identified resource is associated with a respective answer box topic; determining a set of answer box topics from the respective answer box topics; analyzing each answer box topic of the set of answer box topics to identify a first answer box topic that is responsive to the query, the first answer box topic associated with a first search result of the plurality of search results; identifying a first publisher of a first resource identified by the first search result; obtaining an answer box precursor for the first answer box topic using a mapping between a plurality of answer box topics and a plurality of answer box precursors, wherein the mapping includes mapping information provided by the first publisher of the first resource; and providing the search results along with the answer box precursor for the first answer box topic, wherein the answer box precursor includes information that defines an answer box.
24. The system of claim 14 , wherein analyzing each answer box topic of the set of answer box topics to identify the first answer box topic comprises identifying one or more ads associated with the first resource and determining that at least one of the ads is associated with the first answer box topic.