One day, Dick Albright, in the Physics room on the third floor of the building, stood by the window and looked across at a friend of his who was standing at the window of the Chemistry room. The two rooms faced each other across an open space in the back of the building, which was designed to let more light into certain rooms. This space was only open at the third and fourth floors. The second floor was roofed over with a skylight at this point. It was after school hours and Dick was alone in the room. So, apparently, was his friend. Dick raised the window and called across the space to the other boy, who raised his window and answered him. From talking back and forth they passed to throwing a ball of twine to each other. Once Dick failed to catch it, and falling short of the window, it rolled down upon the roof of the second story. Dick promptly climbed out of the window, and sliding down the waterspout, reached the roof and went in pursuit of the ball. One of the windows opening from the third story onto this open space was that in the electric room, and it was under this window that the ball came to a standstill. As Dick stooped to pick it up he found a knife lying beside it. He brought it along with him and climbed back into his room. Then he pulled it out and looked at it. It was an ordinary pocket knife with a horn handle. On one side of the handle there was a plate bearing the name F. Boyd. "Frank Boyd's knife," said Dick to himself. "He must have dropped it out of the window." Idly he opened the blade. It was broken off about half an inch from the point. Dick began to turn things over in his mind. A piece of a knife blade had been found in the electric room. A knife with a broken blade had been found on the roof under the window of the electric room. That knife belonged to Frank Boyd. The inference was very simple. Frank had climbed in the window of the electric room from the roof of the second story and cut the wires, and then climbed out again, and so was not seen coming out of the room into the hall. In climbing out he had dropped the knife without noticing it. He had already left a piece of the blade inside. Frank Boyd was one of the lawless spirits who had caused much of the trouble all through the year. He had also been blackballed at the last election of the Thessalonian Society. It was very easy to believe that he would try to do something to spite the Thessalonians.