It's a stage of development... An inquisitive and curious mind! I agree with peanutter- ask "why do you think....?" you might be surprised at the amazing responses you get. Some will be quite creative and give you even more of an insight into how your DS sees the world and his place in it and other answers will be factually accurate.
Are you planning on your DS going to Kindergarten next year? His curiosity is a great attribute that shows how switched on to learning he is.
If all else fails.... Distraction might also work!!