What things does Lennie do or say that show he is mentally retarded?

1) Lennie carries a dead mouse in his pocket simply to have something to pet.

2) Lennie has to be reminded not to talk to much.... just like a child.

“O.K. Now when we go in to see the boss, what you gonna do?”

“I . . . . I . . . .” Lennie thought. His face grew tight with thought. “I . . . . ain’t gonna say nothin’. Jus’ gonna stan’ there.”

“Good boy. That’s swell. You say that over two, three times so you sure won’t forget it.”

Lennie droned to himself softly, “I ain’t gonna say nothin’ . . . . I ain’t gonna say nothin’ . . . . I ain’t gonna say nothin’.”

3) Lennie is unable to remember things... like a small child.

“No, you. I forget some a’ the things. Tell about how it’s gonna be.”