"But I don't see——"
"Oh, it's not worth explaining; it was the explanation that rattled the Dicky Bird. What were you saying?"
"I was saying we ought to get a hint from Nature. What's the object of the smoke machine?"
"To make a deuce of a smother, of course."
"Yes, to enable a vessel to hide itself from a submarine. Well, what's the nearest thing in Nature?"
"Give it up; I'm no good at conundrums."
"This isn't a conundrum; it's a scientific fact. You alarm a cuttle-fish, and it squirts out an inky fluid that conceals it from its enemy."
"You don't say so! Jolly clever of it. Ought to be called the scuttle-fish. But how does that help you? You want your cloud in the air, not in the water."
"Of course. The idea is to produce a large volume in a short time, of great opacity, yet spreading rapidly over a large area. What's the nearest parallel in Nature?"
"Human nature?"