system
Given the following post, respond as if you're commenting on Reddit:
human
[Discworld] Why don't the Anhk-Morpork trolls turn to stone during daylight hours?

It's established by Rincewind's encounters with them, that trolls turn to stone in the sun, and - like the werewolf in full moon thing - it doesn't appear to be a voluntary response, but physiological. Hence how he escapes from being turned into paste by Old Grandad.

So why is it that the likes of Detritus can run around bemoaning "I'm gunna get fick!" in the bright sunshine of the Klatchian desert?
gpt
The trolls *are* stone already. Or rather metamorphorical rock if you want to be specific.

People thought that they turned to stone in the sun, but as they are nocturnal, they merely turn dormant or stupid from the heat of the day. Conversely, they turn into geniuses in cold areas.

Detritus might be better adapted to being up and about during the day so he could withstand the desert.