"What's that?" Bill and Cayley asked the question simultaneously.
"The key."
"The key?" said Bill.
Cayley lifted his head and looked at Antony. "What about the key?" he asked.
"Well, there may be nothing in it; I just wondered. Suppose Robert was killed as you say, and suppose Mark lost his head and thought of nothing but getting away before anyone could see him. Well, very likely he'd lock the door and put the key in his pocket. He'd do it without thinking, just to gain a moment's time."
"Yes, that's what I suggest."
"It seems sound enough," said Bill. "Sort of thing you'd do without thinking. Besides, if you are going to run away, it gives you more of a chance."
"Yes, that's all right if the key is there. But suppose it isn't there?"
The suggestion, made as if it were already an established fact, startled them both. They looked at him wonderingly.
"What do you mean?" said Cayley.
"Well, it's just a question of where people happen to keep their keys. You go up to your bedroom, and perhaps you like to lock your door in case anybody comes wandering in when you've only got one sock and a pair of braces on. Well, that's natural enough. And if you look round the bedrooms of
