How do babies learn a language nobody explains to them?

Babies learn a language nobody explains by listening first and working out its patterns themselves, step by step, starting with its sounds. Perception always comes before production. From birth to about six months, infants can tell apart the sound contrasts of every human language. After that they tune in to the sounds of the language around them and gradually lose the rest.

Their next problem is harder than it looks: speech has no gaps between words. It's one continuous stream. So how does a baby find the words? Part of the answer is statistics. In the phrase "pretty baby", the syllables "pre" and "ty" turn up together far more often, across everything a baby hears, than "ty" and "ba". In a famous experiment, Jenny Saffran, Richard Aslin and Elissa Newport played eight-month-olds a flat, monotone stream of made-up words like "bidaku", with no pauses at all. After just two minutes, the babies reacted differently to the made-up words than to the same syllables in a scrambled order. They had found the words by tracking which sounds go together.

Children aren't simply copying, either. A toddler may say "gave", then switch to "gived", then go back to "gave", often ignoring corrections. They are building rules, not parroting.

Timing matters. Children exposed to language early almost never fail to learn it, but by around age 12 the window has largely closed, and learning a language like a native gets much harder.