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[He] walked heavily, dragging his feet a little, way a bear drags his paws. His arms did not swing at his sides, but hung loosely. (1.4)
Should we keep count? This is instance #1 of Lennie being compared to an animal—a bear, no less: a massive, occasionally violent creature.
Lennie dabbled his big paw in the water and wiggled his fingers so the water arose in little splashes; rings widened across the pool o the other side and came back again. Lennie watched them go. "Look, George. Look what I done." (1.9)
Lennie doesn't get hands—he gets "paws," and he's fascinated with how those paws can affect the natural world. It's also as though, like an animal, he doesn't quite understand cause-and-effect.
"What you want of a dead mouse, anyways?"
"I could pet it with my thumb while we walked along," said Lennie. (1.36-37)
First, gross. Second, Lennie doesn't seem to have the same hang-ups about death as other people. His mental disability makes him closer to an animal than to a human—which makes us think that Steinbeck is saying that the difference between men and beasts is more of a continuum than a sharp divide.