Why do some children ask why all the time?! My DS is 3 1/2 and his response to almost every sentence anyone utters (including himself) is 'Why?' I'm so over it! Does anyone have any suggestions on how to get past this?

Usually I try to be patient and answer his questions, but sometimes it does my head in. If I turn it around and ask him 'Why?' his response is 'Cos', so I've tried that a bit too - it seems to stop him most of the time, but I feel mean for not answering his questions. I don't want him to feel that he can't ask questions, I just want him to understand when it is appropriate.For example, he might ask me if his sister is asleep. I say 'No, not yet'. He asks 'Why?'... Or, after seeing and playing with his friend at the football, he asks me if Lachie is in his seat. I say yes. He asks why. These aren't very good examples, but they give you a bit of an idea of what I mean. It's almost as if it's just his automatic response to everything!