Messing with them policemen:
‘Well, now, Mr Carstairs, who killed Mountjoy?’
‘To the best of my knowledge,’ said Carstairs, in his precise, dry way, ‘Mountjoy was the victim of an accident.’
‘Is that really your opinion?’ said the inspector.
‘It is— now,’ replied Carstairs quietly.
The two policemen looked at him, but Carstairs merely smiled at them urbanely, and volunteered no further statement.
‘Hum! Thank you, Mr Carstairs,’ said the Chief Constable, hiding his disappointment. Carstairs rose to go.
‘And yet, Mr Carstairs,’ said the inspector, ‘I could swear that yesterday you thought very differently.’
‘A scientist,’ said Carstairs, with his hand on the doorknob, ‘hardly ever thinks exactly alike two days running.’ He nodded cheerfully to them and went out.