Lucy's logic . . .

Mom: "Why did you do that?"
Lucy: "Because I did."

Mom: "Why did you spill that?"
Lucy: "Because I spilled it."

Mom: "Why didn't you take a nap?"
Lucy: "Because I didn't sleep."

Mom: "Why did you hit David."
Lucy: "Because I hit his arm."

Mom: "Why are you afraid of the trampoline?"
Lucy: "Because the trampoline scares me."

You get the idea. And I really haven't figured out a very good rebuttal ;)