It's a wonder how babies learn how to do what they do regularly. For instance, putting fingers into the mouth. And when they are told that they did all this when they are capable of understanding things later on, they laugh it off.
B simply grinned when she was told that she had a baby sister who is naughty. And she didn't know how to react when we were talking about her days as a baby, when she was mischievous herself.
Is it a sudden event which induces a practise and is followed diligently? I don't know. But, it's wonderfully amazing to watch and learn.