Why do trolls turn to stone in sunlight?

Question

Why do trolls turn to stone in sunlight?

Answer 1

Because it says so on JRR Tolkein's artistic license.

He created the trolls in his books, so he gets to say how they act, and how they react to their environment.  Personally, I always thought it was funny that these huge hulking brutes sounded like a couple of little old men squabbling about who gets to have the last bottle of beer.

Answer 2

It's long been known that if you shine a strong light on a republican they will stop what they're doing and stand very still.  They don't actually turn to stone though - they just don't want to be found out as frauds.