"Why does it rain?" "Why can't dogs talk?" "Why does Daddy have to go to work?" Why, why, why.
You try, once, to answer properly. You explain the water cycle. You touch on mammalian vocal anatomy. You begin edging toward household finances. By the third question you notice something: the child is not listening for the answer. Or rather, the answer is not what they came for.
The "why" phase is often treated as a test of parental knowledge and endurance. But the developmental research points in the opposite direction. Give a precise answer and the child's inquiry stops. What is being tested is not your knowledge — it may be your ability to not answer.