Joad rolled the coat up more tightly. "An old turtle," he said. "Picked him up on the road. An old bulldozer. Thought I’d take ‘im to my little brother. Kids like turtles."
– John Steinbeck
The Grapes of Wrath, Chapter 4. Tom Joad tells Jim Casy what he has moving around in his bundled-up coat. In using a metaphor to compare the turtle to a bulldozer, Steinbeck is calling attention to the animal’s determination and tenacity.