My favorite books are the watch books and for some reason I find Detritus very interesting. I was re-reading The Fifth Elephant and I realized that the cold and extremely heavy snow in Bonk seems to have no positive effect on Detritus' intelligence. In fact, he appears as intellectually challenged as he does in Ankh Morpork, referring to "halberds" as "halibuts". How can this be when he gets so much smarter in the cold (even at night in the Klatchian desert). Why didn't he get smart in Bonk?

Good question, but if I remember correctly, the cold in the future pork warehouse - the only occasion when he was really, really intelligent and quick thinking, was so cold it almost killed his fellow colleague. So it seems to have been somewhat chillier than the winter in bonk; also, the cooling helmet still seems to be around, or at least I always presumed it was, so his intelligence level was more or less always the same except under very extreme conditions.
Still, I can't really answer that question, either.

Pratchett does make mention, (on page 144 of my hardback anyhow) when Vimes and Sybil try to stand in front of the Troll's head on the embassy wall that:
"'... It's colder up here, Vimes thought. He's quicker on the uptake.* Even Nobby won't play poker with him in the winter. Damn!'...

* Detritus's silicon-based brain ... etc"
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I agree with Hsing about the helmet and wanted to say that after Detritus got the helmet I don't think, in the general text, Pratchett make so much mention of his low intelligence unless he makes a specific point that is required for the storyline ... but i can't really back that point up without going through every book. I just remembered the quote above as it is a nice scene.

So yes after all that I don't really know (anything at all) either.

R.

Thanks you guys, your replies encouraged me to find out what happened to Detritus' cooling helmet. It gave out in the deserts of Klatch in Jingo (page 273). Then I came across this extract on page 295:

"What do you want me to do with him, Mr Vimes?"
"Detritus?"
The troll saluted with the hand that still held the D'reg.
"All present and correct, sir!"
"But--" And then Vimes realized. "It's freezing cold! Your brain's working again?"
"With rather more efficiency, sir."

This occurred at about three in the morning. And it does say it's freezing cold. Is it possible it's colder at night in the desert than it is in Uberwald? Or does it have something to do with Detritus being from Uberwald originally? I would really appreciate your thoughts on this. Thanks