“One of us, for instance, spoke to a child who called his inflated plastic fish a ‘fis’. In imitation of the children’s pronunciation, the observer said: ‘This is your fis?’ ‘No,’ said the child, ‘my *fis*.’ He continued to reject the adult’s imitation until he was told, ‘That is your fish.’ ‘Yes,’ he said, ‘my fis.’”

The effect has been referred to as the “fis phenomenon” ever since. Such reports indicate that children know far more about adult phonology than their own pronunciation suggests.

How Language Works by David Crystal, page 87 (via linguaphilioist)

My other favourite quote about children’s language learning is about them the futility of overtly correcting children’s language, from McNeill 1966:  

Child: Nobody don’t like me.
Mother: No, say “nobody likes me”.
Child: Nobody don’t like me.
(Eight repetitions of this exchange follow.)
Mother: Now, listen carefully, say “NOBODY LIKES ME”
Child: Oh! Nobody don’t likes me.  

(via allthingslinguistic)