7. A system, comprising: a data storage device; and one or more data processing apparatus that interact with the data storage device and execute instructions that cause the one or more data processing apparatus to perform operations comprising: determining, for a prefix that represents a portion of a search query, one or more completions that yield a completed search query when combined with the prefix, wherein the one or more completions includes a given completion that describes a question; receiving, through a search engine input field presented at a user device, user input including the prefix; identifying, based on the user input, an answer box for the one or more completions, wherein the answer box includes an answer to the question described by the given completion that is included in the one or more completions; and providing, prior to a completed search query being specified by the user input, instructions that cause the answer box to be presented in a display of the user device along with at least some of the one or more completions for which the answer box was identified and the answer to the question described by the given completion, wherein the answer box is a user interface element that presents the one or more completions and the answer to the question described by the given completion while a user enters text input through the search engine input field and before the user submits a search request for the user input.
7. A system, comprising: a data storage device; and one or more data processing apparatus that interact with the data storage device and execute instructions that cause the one or more data processing apparatus to perform operations comprising: determining, for a prefix that represents a portion of a search query, one or more completions that yield a completed search query when combined with the prefix, wherein the one or more completions includes a given completion that describes a question; receiving, through a search engine input field presented at a user device, user input including the prefix; identifying, based on the user input, an answer box for the one or more completions, wherein the answer box includes an answer to the question described by the given completion that is included in the one or more completions; and providing, prior to a completed search query being specified by the user input, instructions that cause the answer box to be presented in a display of the user device along with at least some of the one or more completions for which the answer box was identified and the answer to the question described by the given completion, wherein the answer box is a user interface element that presents the one or more completions and the answer to the question described by the given completion while a user enters text input through the search engine input field and before the user submits a search request for the user input. 9. The system of claim 7 , wherein determining one or more completions that yield a completed search query when combined with the prefix comprises identifying the one or more completions based, at least in part, on a frequency of previously performed searches that included the one or more completions.