What does Slim say he finds funny?
Early in Chapter Three, Steinbeck offers considerable exposition in the form of dialogue between Slim and George. Referring to Lennie, Slim says, "Funny how you an' him string along together." He goes on to explain that very few of the itinerant farm workers travel around together. Then he repeats, "It jus' seems kinda funny a cuckoo like him and a smart little guy like you travelin' together."
Then George goes on to tell Slim about Aunt Clara and about his relationship with Lennie, even going so far as to confide about the trouble the two buddies had just had up in Weed. In Chapter Three the reader learns everything necessary about the "back story"--the events that led up to George and Lennie's arrival at the Salinas River a few miles south of Soledad in Chapter One.
Steinbeck was planning to adapt this novel into a play, and it can be observed throughout the book that he was careful to avoid narration and to rely heavily on dialogue, which would make the adaptation fairly simple. It can also be observed that the characters are frequently introducing themselves to others or introducing other characters to each other. They also call each other by name more frequently than would normally happen in such a milieu. This is for the benefit of the reader and for the future theater audience; and it is especially important because the characters are all men and all of the same social class. The only exception is Curley's wife, and it is noteworthy that she isn't even given a name. She doesn't need to have one for identification purposes.