What are some quotes that show Lennie is mentally challenged in Of Mice and Men?

In Steinbeck's novella the writer uses irony when he gives one of his main characters the last name Small. Lennie is not small. He is a very large and strong man. Steinbeck describes him in contrast to George, the other main character:

Behind him walked his opposite, a huge man, shapeless of face, with large, pale eyes, and wide, sloping shoulders; and he walked heavily, dragging his feet a little, the way a bear drags...

In Steinbeck's novella the writer uses irony when he gives one of his main characters the last name Small. Lennie is not small. He is a very large and strong man. Steinbeck describes him in contrast to George, the other main character:

Behind him walked his opposite, a huge man, shapeless of face, with large, pale eyes, and wide, sloping shoulders; and he walked heavily, dragging his feet a little, the way a bear drags his paws. 

But even though Lennie is very large the name fits him because he has a child like mind. Today he would be considered mentally challenged. In the novel, however, he is described as a crazy bastard, crazy as a wedge, nuts and a cuckoo.

Lennie's personality is mostly described through indirect characterization. In his narration, Steinbeck never outwardly says Lennie is intellectually disabled. Rather, what Lennie says and how the other character's describe him in their dialogue reveals his character. In chapter one, George goes into a tirade about how Lennie's behavior and his obsession with soft things:

“You crazy son-of-a-bitch. You keep me in hot water all the time.” He took on the elaborate manner of little girls when they are mimicking one another. “Jus’ wanted to feel that girl’s dress—jus’ wanted to pet it like it was a mouse—Well, how the hell did she know you jus’ wanted to feel her dress? She jerks back and you hold on like it was a mouse. She yells and we got to hide in a irrigation ditch all day with guys lookin’ for us, and we got to sneak out in the dark and get outa the country. All the time somethin’ like that—all the time. I wisht I could put you in a cage with about a million mice an’ let you have fun.” 

In chapter three when George is talking to Slim he explains how he used to play tricks on Lennie because the big man wasn't smart. He also praises Lennie for being a good worker. He says,

“Jus’ tell Lennie what to do an’ he’ll do it if it don’t take no figuring. He can’t think of nothing to do himself, but he sure can take orders.” 

Later in the book when Lennie goes into Crooks' room the black man exploits Lennie's handicap when he suggests the big man doesn't remember anything he's told. Crooks says,

“A guy can talk to you an’ be sure you won’t go blabbin’. Couple of weeks an’ them pups’ll be all right. George knows what he’s about. Jus’ talks, an’ you don’t understand nothing.” 

In chapter five Curley's wife also comments on Lennie's intelligence but also compliments him when they are talking about petting soft things:

“You’re nuts,” she said. “But you’re a kinda nice fella. Jus’ like a big baby. But a person can see kinda what you mean. When I’m doin’ my hair sometimes I jus’ set an’ stroke it ‘cause it’s so soft.” 

Of course, Curley's wife tempts Lennie too much and the big man ends up breaking her neck when she begins screaming.