Marinette's magic box

 is a small lidded box that Marinette stores her diary in and prevents anyone from stealing it and discovering her secret of being Ladybug.

Appearance
The magic box is a semi-circular chest with a similar shape and color theme as the diary with a keyhole located on the edge. Inside is colored red with an oval-shaped mirror on the top half in the center. The chest comes with a yellow key that Marinette carries with her.

If someone tries to pick up Marinette's diary while the chest is open, it will close and lock, trapping the invader's hand inside. The only way to unlock it is with the key.

Trivia

 * In "Darkblade", Sabrina tries to steal Marinette's diary for Chloé, but the box traps her hand. At the end of the episode, Marinette frees her hand.