M.L.
I also agree with Christy Lee. Another idea is to say, "I don't know. Let's find out!" Then you consult the encyclopedia (otherwise known as the Internet).
As annoying as the why-time can be (and it can be a sort of power game), take it with good humor. One of these days DS will not ask you why - he'll tell you why, because he will have decided he already knows everything important!