What is it about Lennie's actions in the bunk house, which lead Slim to conclude that he has the mental age of a child, and is not deliberately mean or wicked? In Chapter 3

In Chapter Three, Slim has given Lennie a puppy that he's been instructed must stay in the barn with its mother. When Lennie returns to the barn, he goes right to his bunk and turns his back on the other men.... because he's hiding the puppy in his arms. George, of course, knows immediately.... Lennie tries to lie but can't. In response, Slim says;

"Jesus," he said. "He's jus' like a kid, ain't he?"