sentences:  In this sentence, the wh-word doesn't move, and so in the sentence that the child hears, there is no wh-trace.Possible explanations for the eventual acquisition of the notion of empty categories are that the child then learns that even when he or she doesn't hear a word in the original position, they assume one is still there, because they are used to hearing a word.At the beginning of acquisition, children do not have a concrete concept of an empty category; it is simply a weaker version of the concept.