The Daughters’ Staircase was in the ground-floor bedroom of their parents, so Simon always knew the hours of his daughters’ nocturnal comings and goings.
Once the town was terrorized by a series of morbid nocturnal events: people’s chickens and household pets were found mutilated; although the culprit was Crazy Addie, who eventually drowned himself in Barker’s Eddy, people still looked at the Radley Place, unwilling to discard their initial suspicions.
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The mice are nocturnal, so we never see them, but we see their droppings.