17. A computer storage medium comprising executable instructions that, when executed by a processor of a machine, cause the machine to perform operations comprising: receive, from a user, a question prefix representing a portion of a natural language question; selecting, based on the question prefix, at least one index comprising a short text index or a question term index, the short text index comprising a plurality of text entries, each entry corresponding to an associate document, the question term index comprising a plurality of question entries, each entry corresponding to a partial question phrase; search the at least one index and retrieve a plurality of retrieved entries, the plurality of retrieved entries comprising the title of a document or the partial question phrase, or both; select a subset of the plurality of the retrieved entries; create a language model from the subset, the language model comprising a plurality of n-grams each with an n-gram probability; create a plurality of question completion suggestions based on the language model and the question prefix; and return plurality of question completion suggestions to the user via a user interface.
17. A computer storage medium comprising executable instructions that, when executed by a processor of a machine, cause the machine to perform operations comprising: receive, from a user, a question prefix representing a portion of a natural language question; selecting, based on the question prefix, at least one index comprising a short text index or a question term index, the short text index comprising a plurality of text entries, each entry corresponding to an associate document, the question term index comprising a plurality of question entries, each entry corresponding to a partial question phrase; search the at least one index and retrieve a plurality of retrieved entries, the plurality of retrieved entries comprising the title of a document or the partial question phrase, or both; select a subset of the plurality of the retrieved entries; create a language model from the subset, the language model comprising a plurality of n-grams each with an n-gram probability; create a plurality of question completion suggestions based on the language model and the question prefix; and return plurality of question completion suggestions to the user via a user interface. 18. The medium of claim 17 wherein the plurality of retrieved entries each have a metric derived from at least one of: a number of times a document associated with the entry has been viewed; a type of match between the question prefix and the entry; and a number of times a document comes up in search results.