I love Love and Logic's suggested reply to the "But why?" question:
"Why do you think?
If your children are not honestly curious but are just arguing about having their way then it's time to go "brain dead." Use fewer words by simply repeating the reply, "Why do you think? It's better than anger or frustration and sooner or later your child will get tired of your repetitive reponse.
Avoid using, "Because I said so!" But why? Why do you think? (Lol!) Seriously, the reason to avoid that response is because the very words are a command that create a power struggle that invites retaliation.
(This current set of blog posts are all inspired by "Love and Logic Magic When Kids Leave You Speechless," by Jim and Charles Fay.)