How does Cole describe the thinking that keeps him up at night? Hint: it's a simile!

From the text:

“You tossed and turned half the night,”Edwin said.
“Yeah, I couldn’t sleep.”
“Any reason?”
Cole didn’t answer right away.“Just thinking,” he said. When Edwin didn’t reply, he added,“My mind gets to thinking and won’t quit. Like it’s chewing on tough meat. It won’t swallow a thought, and it won’t spit it out. It just keeps chewing it over and over.”