7. A system, comprising: one or more computers programmed to perform operations comprising: while receiving a user input entered in a search engine query input field by a user, and before the user has submitted the user input as a search request: accessing, in a data processing system, historical data, the historical data associating each of a plurality of input-answer box pairs with a respective presentation value, each input-answer box pair associating a text input with an answer box, where a text input is associated with an answer box if the answer box was presented by a search engine in response to a query beginning with an actual input matching the text input, and where the presentation value for each input-answer box pair is derived from a number of times the answer box in the pair was presented by the search engine in response to queries beginning with actual inputs matching the text input in the pair divided by a total number of times queries beginning with actual inputs matching the text input in the pair were received by the search engine; identifying, in the data processing system and from the historical data, one or more candidate input-answer box pairs, where the text input for each candidate pair matches the user input; selecting, by the data processing system, a pair from the one or more candidate pairs, where the pair is selected according to the presentation value for each candidate pair; obtaining content for the answer box in the selected pair; and presenting the answer box to the user.
9. The system of claim 7 , further comprising receiving a plurality of query suggestions for the user input, wherein: presenting the answer box includes presenting a display including the query suggestions and the answer box.