It is established in novels such as The Light Fantastic and Moving Pictures that Discworld trolls turn to stone in daylight, or at least become disabled, and become active at night. This does not seem to happen for Detritus, particularly when he becomes part of the Watch. Why can Detritus function while the sun is up?

My only guess is that he wears a sort of sun block like the trolls Rock and Morry from Moving Pictures.

  • 14
    That would be correct. There's special "cream" or oil trolls put on to be able to walk during the day. It's first mentioned in Moving Pictures and was expensive then. I guess it got cheaper as trolls got more accepted in society.
    – jo1storm
    Commented Sep 29, 2017 at 6:40
  • Have you gotten to Thief of Time and Night Watch?
    – derobert
    Commented Sep 29, 2017 at 14:36
  • @derobert Not yet
    – Neithan
    Commented Sep 29, 2017 at 15:05

4 Answers 4


Cuddy made him a clockwork cooling helmet during Men at Arms which, along with the barrier cream mentioned in Moving Pictures will keep him more active in the heat.

However, (spoiler)

the helmet fails in the Desert in Jingo, once again turning him to stone.

(It gets a bit awkward to lose recurring characters for half the active period of any story unless it's convenient to the plot as per Light Fantastic/Jingo. Pratchett had a tendency to favour the story over consistency, hence trolls turn to stone in Light Fantastic, in later books that's no longer useful.)

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    On a side note... in one of the books Detritus got trapped in a freezer, and got super-smart and started doing complex calculations... he didn't finish them, because he froze solid, and when they thawed him up, he was already too hot (read: stupid) and couldn't remember nor understand what he did. I can't remember which book that was in tho...
    – MBender
    Commented Sep 29, 2017 at 11:54
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    It was in Men At Arms. The scene took place in the Pork Futures Warehouse on Morpork Road.
    – Ian Bush
    Commented Sep 29, 2017 at 13:04
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    @IanBush, importantly, it was the event that caused Cuddy to create the cooling helmet
    – Separatrix
    Commented Sep 29, 2017 at 13:09
  • 1
    Isn't there a tvtropes about things convenient to the plot? Commented Sep 29, 2017 at 14:06
  • 1
    I'd say that the early books are only partly canon for the later books. PTerry has said that his early work wasn't of the same quality as his later work as far as having a plot and so on. As this answer points out, after Guards, Guards / Moving Pictures, canon was pretty consistent. But before that there were lots of things that got thrown out. Commented Sep 29, 2017 at 21:33

Caution : wet paint

When reading Discworld series, There is basically two important stepstones :

  • Equal Rites/Mort.
  • Guards,Guards/Moving pictures.

For each of this stepstones, there is a before and an after.
This is very flagrant in the Light Fantastic : Independent novels linked by Rincewind with full fantasy elements that will never be heard again.
You might even look at the 2 first book as an 'invented story' that either Rincewind or TwoFlowers will later tell.
As a matter of fact, when asking a new reader to start the series, fans frequently advise to start with Mort.

Guards is the first Vimes book, Pictures is the first New Unseen Academy book. Guards is the time Vetinari is really given a character (Pratchett will later admit the previous Patrician is also Vetinari, yet admitting the retcon.. I will try to find the citation).

So basically : Don't try to compare anything from the 3 first book to the rest.
Be tolerant about what you read in the 7 first book to the rest.
The universe was not ready, the paint was wet.

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    This is the real answer. PTerry didn't even try to stay consistent with the first 2 books in lots of ways. I'm glad he didn't try to paper over things he wanted to change, and instead just ignored them and wrote the stories he thought would be good. (And BTW, having recently re-read the first several books in the series, I was surprised how consistent Mort is with later canon. I'd second the recommendation for that as a starting point. Good and consistent plot. Eric is another sort of series of short stories like the first 2 Rincewind books. Fun but skipable.) Commented Sep 29, 2017 at 21:38

Discworld trolls differ from the standard fantasy trolls. While the latter are permanently turned to stone by daylight, the trolls on Discworld are (reversibly) affected by heat: The hotter it gets, the slower their silicon-based brains work, up to a complete freeze by overheating.

Trolls coming to Ankh-Morpork are usually the smarter ones and thus have an edge at resisting the day heat. In addition, heat protections like sun shades or heat blocking creams (as mentioned above by jo1storm) or behaviour adaptions (e.g. working in the night) help. In particular, Detritus wears a helmet with a cooling fan.


Actually, trolls on diskworld are not on heat turning into stone (since they are already made out of stone) - they get more stupid. I guess Pratchett got that idea from PCs and laptops. The hotter a CPU is, slower the PC works, and at high enough temperature it shut down.

In Jingo, Detritus gets stupid during daytime, and smarter during night when it gets cold. Even the cooling helmet didn't help.

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    In The Light Fantastic trolls turning to stone in the sun is fairly i,portante to the plot.
    – Neithan
    Commented Sep 29, 2017 at 11:33
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    @Neithan trolls don't really "turn to stone", they are stones, just clever ones. As long as it's not too hot, anyway.
    – Erik
    Commented Sep 29, 2017 at 11:37
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    @Erik Okay, that’s correct. This answer seems to be saying that they don’t usually get completely disabled, which seems to be incorrect to me based on the books I’ve read so far.
    – Neithan
    Commented Sep 29, 2017 at 11:40
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    Off topic here, but – CPUs don't work slower when they're hot. CPUs always work as fast as the clock demands. Under normal operation, you may be effectively right because when approaching a critical temperature, the frequency governor could need to turn down the frequency to prevent overheating (but is this actually done? In my experience the computer will spin up the fan and only when it gets outright dangerous, shut down everything to prevent meltdown). But that only applies when the heat comes from the processor itself; against heat from the environment, reducing frequency is futile. Commented Sep 29, 2017 at 13:15
  • 2
    It is a characteristic of semiconductors so yes... The hotter they get the "slower" they become. This is seen as an increase in propagation delay. When designing chips such delays are taken into account during their static timing analysis across the operating temp
    – Naib
    Commented Sep 30, 2017 at 10:14

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