"BECAUSE I SAID SO, THAT'S WHY!"
Anyone who has ever dealt with kids has probably said this at some point, or at least heard it when they were kids. I try very hard not to say it to my nieces and nephew because it's a total BS line.
I'll admit that my wife and I have said this on occasion. Sometimes they get you so frustrated that it's all you can say. Other times it's because you don't want to give the real reason because it's an inappropriate subject or something. And occasionally, you just don't know!