one will know the reason why

'Sir, I know what to do,' retorted Mrs Pipchin, 'and of course shall do it. Susan Nipper,' snapping her up particularly short, 'a month's warning from this hour.'

'Oh indeed!' cried Susan, loftily.

'Yes,' returned Mrs Pipchin, 'and don't smile at me, you minx, or I'll know the reason why! Go along with you this minute!'