Um, that's the way we learn
Frequently seen as a sign of verbal indecision or weakness that should be avoided, parents who "um" and "er" in front of their children are in fact likely to be boosting their offspring's mastery of language.
Writing in Developmental Science, University of Rochester researcher Richard Aslin and his colleagues made the discovery by exposing a group of two-and-a-half year old toddlers to a series of images of pairs of objects, one of which was something they were familiar with, and the other an item they had never seen before. Just before the objects were presented, a voice instructed the children to look at one of them. But sometimes the voice included an "ah" or "um" - known in common parlance as a speech disfluency - just before the name of the object; so, for example, the children would hear the instruction "look at 'thee', um, wrench". While this was going on, a computer, running eye-tracking software, scrutinised where the toddlers looked and for how long. What the researchers found was that when an instruction contained such a disfluency for an unfamiliar object, the toddlers looked for much longer - almost 70% of the time compared with 54% normally - at the novel, unknown item on the screen.
This effect was also specific to the two and a half year olds, the group found, because when they tried it with a children under two it didn't work. This strongly suggests, Aslin says, that children rapidly learn that when someone utters a disfluency (um or er), which usually also follows a long "the", prnounced "thee", there will likely follow a word that is more rarely used - hence the delay as the adult searches their mind for it - and which should be attended to because it's probably unknown. This technique, which continues in adulthood and probably also underlies the importance of making appropriate pauses in speech and comedy in order to attentionally-prime an audience, enables the toddler to pick up what's relevant amongst the barrage of novelty with which it constantly assailed.