This research investigates children's knowledge of the syntactic operation that allows the interpretation of telicity in Spanish. In particular, the study investigates whether syntactic operations at the interface between the syntax and the semantics are a cause of children's developmental instability. The data indicates that young children may have difficulties understanding the notion of completion entailed by simple telic predicates (e.g., John filled the bucket) due to the covert object movement operation that these predicates undergo, but have adult-like knowledge when predicates require an application of overt object movement, as in the case of locatum structures (e.g., the water filled the bucket). The results indicate that syntactic operations after Spell-out, that is, at LF, are more difficult to acquire than operations before Spell-out.