A thorough account of how grammar is acquired must handle the problem of how learners deal with covert grammatical elements. In particular, there is cross-linguistic evidence that languages contain verbs that are formed by incorporating a silent grammatical element (a head, in GB terms). Assuming this to be a possibility in natural grammar, this paper investigates what type of input would enable a learner to identify a verb with covert head incorporation, and thus to identify a grammar that contains such a verb. I show that such a grammar cannot be learned from input that does not give the locations of empty heads in sentential structure.
