‘It’s who did it,’ corrected Fraser, fifteen months his junior and ever eager to imply that he was smarter than his big brother.

‘I know it’s who did it,’ Duncan retorted with a tut. ‘God, I knew that when I was a P3. That’s why I’m asking why people talk about a whodunit. What does it mean?’

Amazing how garrulous they became in the middle of the bloody night, Catherine thought. You could ask them a dozen questions throughout the day and receive one-word answers, and as for what was happening at school, it was easier persuading suspects to open up in interviews than to get an expansive answer on that subject. Yet when the time drew near for lights out, or when they had managed to snag a parent in the small hours with the had-a-nightmare routine, they invariably became quite the little interlocutors, keen for all manner of broad discourse. Perhaps she should give it a shot down the cells.

She sighed. Sometimes a bit of chat at this time made them feel they’d got their money’s worth and they’d go back to sleep without a fuss; other times, of course, you just whetted their appetites for more attention.

‘It means a story about a murder, where the detectives have to work out who was responsible.’

‘So why is it not who did it?’ asked Fraser.

‘Don’t the detectives have to work out who did it in every murder?’ enquired Duncan simultaneously.

‘I don’t know why people say “whodunit”.’ She answered Fraser first, knowing that his older brother tended to have a little more patience. ‘I think it was the Americans that coined it.’

‘They spell colour wrong too,’ Fraser observed sagely.

‘Mum,’ Duncan insisted, growing dismayed at Fraser’s success in diverting the conversation.

‘Sorry, pet.’

‘And favourite,’ Fraser continued.

Catherine had to stifle a laugh, aware that she had better give Duncan an answer before he leapt across the gap between the beds and chinned his brother.

‘You’re right. The detectives have to establish who was responsible for a murder, but it’s not always a mystery. When it is a mystery, that’s when the story is called a whodunit.’

‘What about in real life? Is there a name for when it’s a mystery?’

Aye, Catherine thought: a pain in the arse.

‘Do you have to work out who done it, I mean who did it?’ Fraser interrupted again. ‘Is that your job, Mum?’

‘Do you catch murderers?’ Duncan trumped.

‘Have you seen a dead body?’ bid Fraser.

‘Have you ever caught the wrong man?’

Catherine was actually relieved when she heard her mobile ring from her and Drew’s bedroom across the hall, even though it had to be work calling, and even though she was technically still on holiday until tomorrow morning.