Dumbledore chuckled. "The Stone is quite safe, Mr. Weasley," he said. "Voldemort has retreated, thankfully, and with any luck we shan't see him again too soon. And no, Professor Snape," he gave Percy a pointed look, and Percy winced, "was very much on our side. Indeed, he has been monitoring Professor Quirrell all year, in case of just such a scenario as the one that took place this past weekend. It was he who warned me that you and your brothers had become involved."