9. A system comprising one or more computers configured to perform operations comprising: receiving i) data identifying a plurality of resources and ii) query log data identifying queries that result in each resource of the plurality of resources being provided in search results; analyzing the plurality of resources to identify one or more parameters associated with each resource; analyzing, for each resource of the plurality of resources, i) the parameters associated with the resource and ii) the queries that result in the resource being provided in the search results; and based on the analyzing: identifying, for each resource of a subset of the resources of the plurality of resources, an answer box topic from an index of answer box topics, wherein the index of answer box topics stores associations between keywords and answer box topics, the identifying including matching at least one query of the queries that results in the resource being provided in the search results with one or more keywords that are associated with the answer box topic; associating, for each resource of the subset of the resources and based on matching at least one query of the queries that results in the resource being provided in the search results with one or more keywords that are associated with the answer box topic, the respective answer box topic with the resource; and annotating, for each resource of the subset, the resource in an index of resources with the respective associated answer box topic.
9. A system comprising one or more computers configured to perform operations comprising: receiving i) data identifying a plurality of resources and ii) query log data identifying queries that result in each resource of the plurality of resources being provided in search results; analyzing the plurality of resources to identify one or more parameters associated with each resource; analyzing, for each resource of the plurality of resources, i) the parameters associated with the resource and ii) the queries that result in the resource being provided in the search results; and based on the analyzing: identifying, for each resource of a subset of the resources of the plurality of resources, an answer box topic from an index of answer box topics, wherein the index of answer box topics stores associations between keywords and answer box topics, the identifying including matching at least one query of the queries that results in the resource being provided in the search results with one or more keywords that are associated with the answer box topic; associating, for each resource of the subset of the resources and based on matching at least one query of the queries that results in the resource being provided in the search results with one or more keywords that are associated with the answer box topic, the respective answer box topic with the resource; and annotating, for each resource of the subset, the resource in an index of resources with the respective associated answer box topic. 11. The system of claim 9 , wherein, for each resource, the parameters includes analyzing metadata associated with the resource, wherein the answer box topic for the resource is identified based on the metadata associated with the resource.