According to U.S. News and World Report, babies who stick out their tongue or open their mouth are not mimicking their parents or an adult, even if the adult does it and the baby follows suit. At least, not until they are at least a couple of months old.
"Infants aren't born with the ability to copy what other people do, but they acquire that skill during the first months of life," said Virginia Slaughter, the lead researcher of the study from Queensland, Australia.
In order to take a closer look at how babies react to a funny face or change in facial expression, Slaughter closely examined the reactions to facial expressions of over 106 babies ranging from 1-9 weeks old. The Daily Mail reports that a wide variety of expressions such as tongue-movements and open mouths were used, along with vocal sounds such as clicks and squeals.
The babies would react, of course, but it was just as likely that they would use a different facial expression when reacting to the adult's silly face. While it is still unclear why the babies take nearly two months to develop imitation skills, in contrast the parents were found to imitate their child or children on more than one occasion.
'In another study from our lab, we found that parents imitate their babies once every two minutes on average," said Slaughter. "This is a powerful means by which infants can learn to link their gestures with those of another person."