1. A method for guiding a user in building a query comprising: providing an interaction space in which a user formulates a query using a plurality of query building blocks selected from a predefined set of query building blocks, wherein selectable ones of the query building blocks are displayed in the interaction space; providing a content-based mechanism which limits the selection of the next building block to those which will yield at least one result from an indexed collection of documents, a structure-based selection mechanism which limits the selection of the next building block to those which meet a predefined grammar, and a usage-based selection mechanism which proposes a next building block based on patterns of building blocks used in building prior queries; with a processor, guiding the user's formulation of the query including, when at least one building block has been selected by the user for formulating the query, guiding selection of at least a next building block by at least one of: the content-based mechanism which limits the selection of the next building block to those which will yield at least one result from the indexed collection of documents, the structure-based selection mechanism which limits the selection of the next building block to those which meet the predefined grammar, and the usage-based selection mechanism which proposes the next building block based on patterns of building blocks used in building prior queries, wherein the interaction space includes a question editing area and wherein the method comprises providing for the user to move selected blocks into the question editing area in formulating the query.