“How on earth⁠—”

Antony laughed happily and sat down on the sofa beside him.

“You don’t really want it explained,” he said, smacking him on the knee; “you’re just being Watsonish. It’s very nice of you, of course, and I appreciate it.”

“No, but really, Tony.”

“Oh, my dear Bill!” He smoked silently for a little, and then went on, “It’s what I was saying just now⁠—a secret is a secret until you have discovered it, and as soon as you have discovered it, you wonder why everybody else isn’t discovering it, and how it could ever have been a secret at all. This passage has been here for years, with an opening at one end into the library, and at the other end into the shed. Then Mark discovered it, and immediately he felt that everybody else must discover it. So he made the shed end more difficult by putting the croquet-box there, and this end more difficult by⁠—” he stopped and looked at the other “by what, Bill?”