"And she provided him besides with a ball of thread,
bidding him to fasten the end of it to the entrance
of the Labyrinth, and unwind it as he went in, that
it might serve him as a clue to find his way out again."

— "Theseus and Ariadne," by Charles Morris

From "Ariadne's Clue," a post of March 1 last year—

http://www.log24.com/log/pix11/110301-DeathlyHallows759.jpg

The Watson here is not Emma, but Victor—

http://www.log24.com/log/pix12/120316-Watson7detail.jpg