1. A method performed by data processing apparatus, the method comprising: receiving a query from a user device; receiving data identifying a plurality of resources determined to be responsive to the query; accessing an index of answer box topics, the index storing associations between resources and answer box topics; based on the accessing, analyzing the plurality of resources to identify a subset of resources of the plurality of resources that are annotated with a respective answer box topic, the answer box topic for each resource of the subset of resources based on content of the respective resource; determining a set of answer box topics from the respective answer box topics associated with the subset of resources; analyzing each answer box topic of the set of answer box topics to identify a first answer box topic that is i) responsive to the query and ii) based on the content of the respective resource the first answer box topic annotates; 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 answer box precursor includes information that defines an answer box; and providing, to the user device, the answer box precursor for the first answer box topic in response to the query.
1. A method performed by data processing apparatus, the method comprising: receiving a query from a user device; receiving data identifying a plurality of resources determined to be responsive to the query; accessing an index of answer box topics, the index storing associations between resources and answer box topics; based on the accessing, analyzing the plurality of resources to identify a subset of resources of the plurality of resources that are annotated with a respective answer box topic, the answer box topic for each resource of the subset of resources based on content of the respective resource; determining a set of answer box topics from the respective answer box topics associated with the subset of resources; analyzing each answer box topic of the set of answer box topics to identify a first answer box topic that is i) responsive to the query and ii) based on the content of the respective resource the first answer box topic annotates; 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 answer box precursor includes information that defines an answer box; and providing, to the user device, the answer box precursor for the first answer box topic in response to the query. 4. The method of claim 1 , wherein the mapping includes mapping information provided by a publisher of a resource associated with the first answer box topic, the method further comprising receiving the answer box precursor for the first answer box topic from the publisher.