I have two kids, and the youngest is just starting on the 'why?' phase. Its great fun! I treat it as a game to try and see how long I can keep on answering. As soon as I don't know the answer, I explain that I don't know (or perhaps that nobody yet knows) and discuss how we could find out.
One interesting thing I discovered - a surprisingly large number of things will either lead to physics or human psychology if you keep asking 'why?'. However I don't know if that reflects some deeper truth about the world, or just my bias in how I answer the questions.
One interesting thing I discovered - a surprisingly large number of things will either lead to physics or human psychology if you keep asking 'why?'. However I don't know if that reflects some deeper truth about the world, or just my bias in how I answer the questions.