Theoretical arguments for considering production as a source of input for analysis (the OUTPUT-AS-INPUT hypothesis) are reviewed, and empirical evidence supporting this hypothesis is presented. The evidence consists of a longitudinal study of the developmental course of a self-created form, produced by one Dutch child. This form is the product of blending two words, wat and iets, to yield unitary wat-iets. In Dutch, independent wat and iets may each mean ‘some’ and/or ‘something’. Though wat-iets is not permitted by the language, and so does not occur in environmental input, the form stays in the child's repertoire for about 10 months (between 3;8 and 4;7) and is apparently subjected to processes of generalization: first the child treats wat-iets as a two-word frame that may be regularized, later as a unitary word that may be semantically extended. After the extension of wat-iets, independent synonymous wat and iets appear for the first time in the child's speech. It is argued that the child actually analysed his own creation.