What do they know?
They stumble around making babbling baby noises, running into furniture, and falling down. Next thing, they’re speaking a language. They don’t know subjects and predicates or present tense and past tense. They don’t study. They don’t know how to spell or even how to write. They just listen and repeat and start talking until it sounds right. They don’t have any idea how hard it is, or how long it takes, to learn a new language. Live in a multi-lingual house? No problem, they just learn two languages at once!
Kids.