The marauders used to take turns taking care of Harry when the others had Order work, or too busy or needed a night-off. It became a tradition among them, as they were passing the baby into the next caretaker's hands, to say: "You're it. Good luck."

The last thing Sirius saw before falling through the veil was Remus running over to Harry, and the last thought that ran through his head was: "You're it, good luck"