"Is it fine then?"
"No, it rains."
"You surprise me. But suppose it had started by being low pressure at B?"
"Why, then of course it would have been raining the whole time at B."
"Where would A have got its rush of air from then?"
"From the place C."
"Is it fine there?"
"No, it's raining. It is like B was after the air rose at A."
"Oh. Then whatever happens at these places, A, B and C, it must rain."
"More or less, yes. More really."