Based on a ficlet sent my way by velvet(velcro)heart awhile ago.
The door opened quietly on oiled hinges, too quietly for the tiny, fully-engrossed little filly to notice. With her face screwed up in utter concentration, she glared at her reflection in the mirror, the struggling violet glow of her magic levitating a slender, marked ribbon to the tiny stub of her still-youthful horn.
So focused on the tape-measure was she, that not even the soft clop of golden horseshoes upon the carpeted floor of her room warned her, until the sudden 'What are you doing, my faithful little student?' sent her and her measuring instrument cascading to the floor in a sudden, swiftly-entangled ball of filly and ribbon.
"P-princess! I wasn't trying to see how much bigger your horn was, honest!" the little filly laying at Celestia's forehooves quickly denied.
"Oh?" Celestia's magic gently grasped the tape-measure, untangling her little student while conjuring a second measure, "You weren't? Well, that just means the question remains unresearched. Would you like a little help?"
"You would - with me? I mean, I know I'm smaller, I won't ever be as long as you, but..."
"Why don't you come up here in front of the mirror with me, hmm?"
Two mares soon found themselves sharing a reflection, one ancient, one fresh and young, one barely expending effort, the other focusing as hard as she could to keep her tape-measure hovering near her tiny stub of a horn.
Sneaking a peek to make sure the little filly was too focused to notice, Celestia sneakily kinked her tape-measure twice, and pushed the two kinks within touching distance of each other. "Oh dear, look at that, you're already bigger than I am!" the princess announced in mock-shock.
Twilight Sparkle blinked, looking at her measure, then at the Celestia's kinked one, before sneaking a glance at the actual horn in question. Even the little filly couldn't hope to miss that big of a discrepancy, and a little giggle escaped, "Oh, princess, you're cheating!"
"Oh dear, looks like you've caught me! Well... I may have been exaggerating a teensy little bit..." Celestia shrank the distance between the two folds on her tape-measure just a bit further. "There you go!"