Why don't you prove it?

In the Harry Potter movie, Chamber of Secrets, when Harry accuses Malfoy's father of slipping Riddle's diary into Ginny's possesions, Mr. Malfoy says, "Why don't you prove it?" but he delivers the line exceptionally well.

My daughter and I sometimes use that line in banter:

Me: "You ate the last piece of chocolate!"

Daughter: "Why don't you prove it?"

Well, now I have proof!

This is my proof copy of Rose of Par Kluhnd. I'm going over it now, getting ready for the print version to become available. I laughed when I saw this on the very last page. I showed it to my daughter the first time she said, "Prove it!" after I'd gotten it in the mail.

She said, "Gaaah!" in frustration, claiming it wasn't fair.

Mwah ha ha! Too bad! Now I have proof.