`What is Scrape?' Vimes said, leaning back in the room's one spare chair and staring at Brick as a zoologist might eye a fascinating but highly unpredictable new species. He'd put the stone ball from the mysterious Mr Shine on the table by the bowl, to see if it got any reaction, but the troll paid it no attention.

`Scrape? You don't see it much dese days now dat Slab's so damn cheap,' rumbled Detritus, who was watching his new find with a proprietorial air, like a mother hen watching a chick who was about to leave the nest. `It what you "scrape up", see? It a few bits o' draingrade Slab boiled up in a tin wi' alcohol and pigeon droppin's. It what der street trolls make when dey is short o' cash an'. .. What is it dey's short of, Brick?'

The moving spoon paused. `Dey is short o' self-respect sergeant,' he said, as one might who'd had the lesson shouted into his ear for twenty minutes.

`By Io, he got it!' said Detritus, slapping the skinny Brick on the back so hard that the young troll dropped his spoon in the steaming gloop. `But dis lad has promised me all dat is behind him and he is damn straight now, on account o' havin' joined my one-step programme! Ain't dat so, Brick? No more Slab, Scrape, Slice, Slide, Slunkie, Slurp or Sliver for dis boy, right?'

`Yes, sergeant,' said Brick obediently.

`Sergeant, why do the names of all troll drugs start with ess?' said Vimes.

`Ah, it make dem easier to remember, sir,' said Detritus, nodding sagely.

`Ah, of course. I hadn't spotted that said Vimes. `Has Sergeant Detritus explained to you why he calls it a one-step programme, Brick?'

`Er... 'cos he won't let me put a foot wrong, sir?' said Brick, as if reading it off a card.

`An' Brick here's got something else to say to you, haven't you, Brick?' said the maternal Detritus. `Go on, tell Mister Vimes.'

Brick looked down at the table. `Sorry I tried to kill you, Mrs Vimes,' he whispered.

`Well, we'll see about that, shall we?' said Vimes, for something better to say. `By the way, I think you meant Mister Vimes, and I prefer it if only people who've fought alongside me call me Mister Vimes.'

`Well, technic'ly Brick has fought-' Detritus began, but Vimes put down his coffee mug firmly. His ribs were aching.

`No, "in front of" is not the same thing as "alongside"; sergeant,' he said. `It really isn't.'

`Not really his fault, sir, it was more a case o' mis-takeniden-tity' Detritus protested.

`You mean he didn't know who I was?' said Vimes. `That didn't seem to-'

`Nosir. He didn't know who he was, sir. He thought he was a bunch o' lights and fireworks. Trust me, sir, I reckon I can make something o' this one. Please? Sir, he was out o' his brain on Big Hammer and still he was walkin' about!'

Vimes stared at Detritus for a moment and then looked back at Brick.

`Mr Brick, tell me how you got into the mine, will you?' he said.

`I told the other polisman-' Brick began.

`Now you tell Mister Vimes!' growled Detritus. `Right now!'

It took a little while, with pauses for bits of Brick's mind to shunt into position, but Vimes assembled it like this:

The wretched Brick had been cooking up Scrape with some fellow gutter trolls in an old warehouse in the maze of streets behind Park Lane, had blundered down into the cellar looking for a cool place to watch the display, and the floor had given way under him. By the sound of it he'd fallen a long way, but to judge from the troll's natural state he probably floated down like a butterfly. He'd ended up in a tunnel `like a mine, y'know, wi' all wood holdin' the roof up' and had wandered along it in the hope that it would lead back to the surface or something to eat.

He didn't start to worry until he came out into a far grander tunnel and the word `dwarfs' finally reached a bit of his brain with nothing to do but listen.

A troll in a dwarf mine goes on the rampage. It was one of those givens, like a bull in a china shop. But Brick seemed refreshingly free of hatred towards anyone. Provided the world supplied enough things beginning with `S' to make his head go `bzzz!' - and the city had no shortage of these - he didn't much care about what else it did. Brick, down in the gutter, had dropped below even that horizon. No wonder Chrysoprase's shakedown hadn't corralled him. Brick was something you stepped over.

It might even have occurred to Brick, standing there in the dark with the sound of dwarf voices in the distance, to be afraid. And then he'd seen, through a big round doorway, one dwarf hold up another and hit it over the head. It was cave-gloomy, but trolls had good night vision and there were always the vurms. The troll hadn't made out details and was not particularly looking for them. Who cared what dwarfs did to one another? So long as they didn't do it to him, he didn't see a problem. But when the dwarf that had done the bashing started to shout, then there was a problem, large as life.

A big metal door right by him had slammed open and hit him in the face. When he peered out from behind it, it was to see several armed dwarfs running past. They weren't interested in what might

be behind the door, not yet. They were doing what people do, which is run towards the source of the shouting. Brick, on the other hand, was interested only in getting as far away from the shouting as possible, and right here was an open door. He'd taken it, and run, not stopping until he was out in the fresh night air.

There had been no pursuit. Vimes wasn't surprised. You needed a special kind of mind to be a guard. One that was prepared to be in a body that stood and looked at nothing very much for hours on end. Such a mind did not command high wages. Such a mind, too, would not be likely to start a search by looking in the tunnel it had just arrived by. It would not be the sharpest knife in the drawer.

And so, aimlessly, without intent, malice or even curiosity, a wandering troll had wandered into a dwarf mine, spotted a murder through a drug-raddled perception, and wandered out again. Who could plan for anything like that? Where was the logic? Where was the sense?

Vimes looked at the watery, fried-egg eyes, the emaciated frame, the thin dribble of gods-knew-what from a crusted nostril. Brick wasn't telling lies. Brick had enough trouble dealing with things that weren't made-up.

`Tell Mister Vimes about the wukwuk,' Detritus prompted.

`Oh, yeah,' said Brick. `Dere was dis big wukwuk in der cave.'

`I think I'm missing a vital point here,' said Vimes.

`A wukwuk is what you make wi' charcoal an' nitre an' Slab,' said the sergeant. `All rolled up in paper like a cigar, you know? He said it was-'

`We call dem wukwuks 'cos dey looks like ... you know, a wukwuk,' said Brick, with an embarrassed grin.

`Yes, I'm getting the picture,' said Vimes wearily. `And did you try to smoke it?

'Nosir. It was big,' said Brick. `All rolled up in their cave, jus' by the manky of tunnel I fell into.'

Vimes tried to fit this into his thinking, and left it out for now. So

... a dwarf did it? Right. And, at that moment, he believed Brick, although a bucket of frogs would make a better witness. No sense in pushing him further right now, anyway.

`Okay,' he said. He reached down and came up with the mysterious stone that had been left on the floor of the office. It was about eight inches across, but curiously light. `Tell me about Mr Shine, Brick. Friend of yours?'

`Mr Shine is everywhere!' said Brick fervently. `Him diamond!' `Well, half an hour ago he was in this building,' said Vimes. `Detritus?'

`Sir?' said the sergeant, a guilty look spreading across his face. `What do you know about Mr Shine?' said Vimes. `Er ... he a bit like a troll god ..: Detritus muttered.

`Don't get many gods in here, as a rule,' said Vimes. `Someone's