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While quietly walking my small dog, we often pass houses where the dog inside, which is out of sight, begins barking. How does the dog know to bark if it can't see, smell or hear us?

I have an oak tree in a pot grown from a seedling. After three years it is healthy but barely a metre tall. However, its leaves are much the same size as those of a mature oak. In animals, a stricter law of proportion seems to apply: babies have tiny hands and kittens tiny claws. Why this difference?