“He was opening the window!” cried Antony.

“Who?”

“Cayley, of course.” Very gravely and slowly he expounded. “He came in here in order to open the window. He shut the door so that I shouldn’t hear him open the window. He opened the window. I came in here and found the window open. I said, ‘This window is open. My amazing powers of analysis tell me that the murderer must have escaped by this window.’ ‘Oh,’ said Cayley, raising his eyebrows. ‘Well,’ said he, ‘I suppose you must be right.’ Said I proudly, ‘I am. For the window is open,’ I said. Oh, you incomparable ass!”

He understood now. It explained so much that had been puzzling him.