Troll's clothes?

[Hot, crispy nice hobbit]

Heh folks, I am in one of my craze again...

Here's the riddle: why do the clothes trolls wear turn into stone with them as well?

He held out a largish key, though no doubt William had thought it very small and secret. It must have fallen out of his pocket, very luckily, before he was turned to stone.

Bilbo tried to pickpocket the William the troll, and the troll's purse squeaked. After a fuzz that followed, not excluding Gandalf's awesome display of ventriloquism, Bilbo picked up the key which dropped from William's pocket; before he turned into stone together with his clothes!

Are troll's clothes merely skin? Does that mean that William had a pocket in his skin like a kangaroo?

I am all confused in this.