Food (or gum) for thought.

After spending his break chatting with the local waitress, the truck driver must eventually return to the road, but before he goes, he "peel[s] the wrapper from a piece of gum" and while "holding the stick of gum in front of his lips" suggests that the waitress shouldn't do anything (sexually) that she wouldn't want him to find out about.

The peeling of the wrapping paper is clearly suggestive of the peeling off of a woman's clothing and holding the gum before his mouth is implicit of temptation and delayed gratification (emotions that a truck driver would be well-acquainted with).

After he leaves the diner, the driver is temporarily alone with his thoughts before meeting up with Tom Joad. It can be assumed that his thoughts are still on the waitress as he "gnawed down on the stick of gum slowly, opening his jaws and lips wide with each bite. He shaped the gum in his mouth, rolled it under his tongue..." This kind of language is useful in describing the breaking-in of a new piece of gum, but when read within the context of the surrounding paragraphs, it is easy to see the sexually-charged nature of Steinbeck's carefully worded phrases.

After Joad climbs into the truck, the driver examines him before striking up a conversation, simultaneously chewing his gum. "He chewed as though thoughts and impressions were being sorted and arranged by his jaws before they were finally filed away in his brain." Later, the driver experiences a moment of awkward anxiousness after he fears he has offended Joad. "The driver chewed as rythmically, as thoughtfully, as a cow. He wanted to let the whole emphasis of the preceeding passage disappear and be forgotten."

Rather than blatantly explaining his characters' thoughts, John Steinbeck has chosen a more roundabout technique. By portraying thoughts through actions (the gum), Steinbeck is being expressively implicit and creating a stronger narrative; one that is uninterrupted by the all-too-obvious "Joad thought" or "the driver secretly hoped."