Because I said so.

The four words that I swore I would never ever say to my future children.

And yet I don't even have my own kids, but find myself saying it to the kids I teach.

Teacher why can't I hit my friend?

Because I said so.

Teacher why can't I throw this toy?

Because I said so.

Teacher why can't I do this?

Because I said so.

Teacher why can't I do that?

Because I said so.
When you think about it, it really is a valid reason :)
......Because I said so.