'"Why, mother?"
'"Because!"'
I came across the above passage near the beginning of one of Myrtle Reed's stories—The Master's Violin—and, towards the end, I found this:
'"Iris, I have been miserable ever since I told you I wrote the letters."
'"Why, dear?"
'"Because!"'
And then, in quite another book—Maurice Thompson's Sweetheart
Manette—I came upon this:
'"Why can't you tell me?" asked Rowland Hatch.
'"I don't know that I have the right," replied Manette.
'"Why?"