Meltzoff and Moore reported experiments showing that human newborns possess the ability to imitate certain facial expressions, such as a protruding tongue or an open mouth. Yet numerous researchers challenge Meltzoff's interpretation that such neonatal imitation is the origin of later imitation, which appears at approximately eight to twelve months of age. These researchers point out that the neonatal imitative response disappears or is lessened at approximately two months. Moreover, since in follow-up studies only one type of imitative response toward a facial expression (that is, tongue protrusion) was observed, some researchers assert that neonatal imitation is not imitation at all but may simply be a form of exploratory behavior in response to interesting stimuli.