Hahahaha! I have two that do this all day long...especially a 3 year old and my 2.5 year old "Why are you changing his diaper?" me: "because he pooped" dcg: "Why?" LOL
It is just a part of kids development to ask questions, even when they know the answers. If I know that they know the answer, I turn it back on them "You tell me" or "What do you think?". It can help...although my 3 year old dcg will say "Just because?" when I ask her...I think that is the standard answer her parents give...LOL. She has a really hard time answering whenI turn it back on her, but I am working on that with her as it is a thinking skill I believe is valuable. As annoying as the questions are, I try to take them as an opportunity to get kids thinking and answering some things for themselves. I know how you feel though...sometimes I feel like tearing my hair out too!