They tried to be too clever ”and that was their undoing." Poirot puffed at his tiny cigarette, his eyes fixed on the ceiling. 
Poirot was staring at the ceiling because he was lost in thought. 
mnli hypothesis: Poirot was staring at the ceiling because he was lost in thought.  premise: They tried to be too clever ”and that was their undoing." Poirot puffed at his tiny cigarette, his eyes fixed on the ceiling. 