  In the case in which the relevant form has never been  encountered before in the data, however, the child is better off: the fact that she has never heard any utterance with the structure of  *Is that girl who in the jumping castle is Kayley's daughter or *Is that mess that that on the floor in there is yours?  is evidence that strings of that type are not sentences. Again, the  evidence is not decisive, and the child should be prepared to revise  her grammar should strings of that kind start appearing. Nonetheless, the non-occurrence of a string, suitably interpreted in the light of  other linguistic information, can constitute negative evidence and  provide learners with reason to reject overgeneral grammars.