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; 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: 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.