"Ask him--do," urged 'Phemie, at last curious enough to have Lyddy share all the mystery that had been troubling her own mind since they first came to Hillcrest. "I'll do so the very first time I see him," declared Lyddy. But something else happened first--and something that brought the mystery regarding Professor Lemuel Judson Spink to a head for the time being, at least. 'Phemie lost the key to the green door! Now, off and on, that missing key had troubled Lyddy. She had seldom spoken of it, for she had never even known it had been in the door when the girls came to Hillcrest. Only 'Phemie, it will be remembered, had the midnight adventure in the old doctor's suite of offices in the east wing. Lyddy only said, occasionally, that it was odd Aunt Jane had not sent the key to the green door when she expressed all the other keys to her nieces when the project of keeping boarders at Hillcrest was first broached. At these times 'Phemie had kept as still as a mouse. Sometimes the key was worn on a string around her neck; sometimes it was concealed in a cunning little pocket she had sewn into her skirt. But wherever it was, it always seemed--to 'Phemie--to be burning a hole in her garments and trying to make its appearance. After finding Professor Spink filling the bottles with water up by Pounder's Brook, the girl was more than usually troubled about the east wing and the mystery. She moved the key about from place to place. One day she wore it; another she hid it in some corner. And finally, one night when she came to go to bed, she found that the cord on which she had worn the key that day was broken and the key was gone. She screamed so loud at this discovery that her sister was sure she had seen a mouse, and she bounded into bed, half dressed as she was. "Where--where is it, 'Phemie?" she gasped, for Lyddy was as afraid of mice as she was of rats. "Oh, mercy me!" wailed 'Phemie, "that's what I'd like to know." "Didn't you see it?" cried her trembling sister. "It's gone!" returned 'Phemie. Lyddy got gingerly down from the bed. "Then I'd like to know what you yelled so for--if the mouse has disappeared?" she demanded, quite sternly. And then 'Phemie, understanding her, and realizing that she had almost given her secret away, burst into a hysterical giggle, which nothing but Lyddy's shaking finally relieved. "You're just as twittery as a sparrow," declared Lyddy. "I never _did_ see such a girl. First you're squealing as though you were hurt, and then you laugh in a most idiotic way. Come! do behave yourself and go to bed!" But even after 'Phemie obeyed she could not go to sleep. Suppose somebody picked up that key? She had no idea, of course, where it had been dropped. Certainly not on the floor of her bedroom. Some time during the day, inside, or outside of the house, the key, with its little brass tag stamped with the words "East Wing," had slipped to the ground. Now--suppose it was found?