II

“I don’t mean only that. I mean all of us⁠—we say things that aren’t really so, and never know that we’ve done so. For instance, both you and I, without doubt, have said some time or other, ‘There’s the post,’ when what we really meant was that we’d heard a double knock and the rattle of the letter box. Nine times out of ten we’d be right, and it would be the post, but just possibly the tenth time it might be only a little urchin playing a joke on us. See what I mean?”

“Ye‑es,” said Mr. Marvell slowly. “But I don’t see what you’re driving at?”

“Don’t you? I’m not sure that I do myself. But I’m beginning to see. It’s like the stick, Tuppence. You remember? One end of it pointed one way⁠—but the other end always points the opposite way. It depends whether you get hold of it by the right end. Doors open⁠—but they also shut. People go upstairs, but they also go downstairs. Boxes shut, but they also open.”

“What do you mean?” demanded Tuppence.

“It’s so ridiculously easy, really,” said Tommy. “And yet it’s only just come to me. How do you know when a person’s come into the house? You hear the door open and bang to, and if you’re expecting anyone to come in, you will be quite sure it is them. But it might just as easily be someone going out .”

“But Miss Glen didn’t go out?”

“No, I know she didn’t. But someone else did⁠—the murderer.”

“But how did she get in, then?”

“She came in whilst Mrs. Honeycott was in the kitchen talking to Ellen. They didn’t hear her. Mrs. Honeycott went back to the drawing-room, wondered if her sister had come in and began to put the clock right, and then, as she thought, she heard her come in and go upstairs.”