Your piss ignites the straw. Apparently, it's highly acidic. Smoke and flames rise around you in the cage, and you panic. Until you realize that you have grown to fit the cage, and your skin isn't being burned.
"What happen me?" you ask.
"Alright, now you are a troll. Trolls can only be destroyed by sunlight. So you stay in the cave except at night, and have to be back inside before sunrise. Understand?"
You squat in the cage for a few minutes thinking while the witch taps her foot and rolls her eyes.
"Think so?" you say uncertainly.
"Don't know why I thought that transforming a human into a troll would give me a smarter troll," the witch said with a sigh. "Wait, it did speak in short sentences, my old troll only speaks in single words." She sounded happy.
You scratch your stone ass with your stone fingers, and it makes an awful grinding sound. The witch grits her teeth, and spins around and disappears deep inside the cave.
As you squat in your dimly lit cage, your eyes adjust to the darkness. The torches are unnecessary for trolls, who can see in the dark. They must serve only to guide the witch. You hear a distant thumping, and screaming human voices slowly approaching from outside the cave. A huge troll ducks its head to enter the cave. The huge troll is carrying two or three struggling humans in each of its massive hands. It sniffs the air, and heads toward you. It offers the hand filled with three struggling humans toward you in the cage and asks, "Food?"
Your stomach rumbles, you are hungry.