[Rarity is a little bit insulted by this lack of trust. She crosses her arms.]

Only if you will promise never to lie to me or to anyone again. Don't give me a reason not to trust you. [She frowns, giving AJ a scolding look. That really bothered her; the lying.]

And we're best friends, Applejack. Of course I can keep a secret. Now, what is it? Surely it can't be as bad as you're making it out to be...