To answer this question, we first need to establish what the main idea of Me Talk Pretty One Day is. This collection of humorous essays has a theme of language, vocabulary and vernacular running the whole way through. In "You Can't Kill the Rooster," for example, Sedaris comments on his brother's regular use of swear words. The titular essay, "Me Talk Pretty One Day," deals with Sedaris's struggles in learning French when living in Paris. In "Go Carolina," Sedaris tells readers about a school speech therapist's attempts to rid him of his lisp. It is therefore evident that we are looking for a metaphor that deals with language and speech.

The next matter we need to clarify is the definition of a metaphor. In simple terms, it is a phrase figuratively applied to something that it cannot be literally applied to. To put it another way, it is a figure of speech that helps to explain an idea without being literally true.

I would argue that the metaphor which best describes the main idea is when Sedaris is talking about his speech therapy, and describes the first of many times that his teacher would correct his speech as "the first battle of my war against the letter s." The battle and war that he refers to here are not physical struggles as the words imply. The words allude to Sedaris's personal struggle to speak clearly.

This metaphor captures the main idea of Me Talk Pretty One Day by encouraging the reader to think about speech and language.

The narrator of “Me Talk Pretty One Day” uses the metaphor of a “lazy tongue” to describe his not very successful efforts to pronounce French. Of course, his tongue isn't literally lazy; that's why it's a metaphor. But it does seem to the hapless narrator that his tongue has gone lazy on him as he tries to get it around an entirely new set of words in a foreign language he's struggling to learn.

A tongue doesn't just refer to a part of the body; it's another name for a language. So the narrator's use of the “lazy tongue” metaphor has a double meaning. His French is lazy in that it appears to show a marked lack of effort on his part.

The narrator also uses a similar metaphor in describing other members of his family. For instance, his sisters Amy and Gretchen are undergoing treatment for their “lazy eyes”. His other sister Lisa was born with a “lazy leg”. And his father always used to accuse the narrator's mother of having a “lazy mind”. If this accusation is true, then perhaps it would account in some way for the narrator's inability to learn the French language. Maybe his painful language struggles have a genetic foundation.

David Sedaris uses the following metaphor in the first chapter of his book to tell a story about his speech impediment during his childhood. Sedaris writes:

"It was the first battle of my war against the letter s, and I was determined to dig my foxhole before the sun went down."

This metaphor describing Sedaris’s trouble with speech is an example of how Sedaris struggles to fit in within the norms of society. The short stories of this book, which are filled with humor and irony, focus on the central theme of being an outsider. As a child, Sedaris becomes a bit of an outsider in North Carolina when he is forced to participate in speech therapy. Even within Sedaris’s own family, Sedaris writes that he is different from his father: Sedaris has the creative imagination while his father has a more scientific approach to life.

Sedaris later feels like an outsider once again as an adult when he moves to France and doesn’t have the ability to speak the French language. Sedaris also discusses his struggles to adjust to French customs and express his unique sense of humor in France.

Sedaris uses the following metaphor to describe the functioning of his French class:

That's the way they do it here—everyone into the language pool, sink or swim.

In this metaphor, he likens the way his language class works to the way a swim class works: people must jump into the pool whether they are going to sink or swim. In Sedaris's French class, he is forced to speak French all the time and to understand his somewhat satirical teacher without the skill or confidence to speak or understand French. In this sense, his class is similar to that of a swim class in which students have to jump into the pool and immerse themselves in the water, even if they don't know what they are doing. The idea is that they will learn by doing and develop skills and confidence, even if they feel like they will drown at first.

While "Me Talk Pretty One Day" has many metaphors that touch on the main idea of David Sedaris feeling out of place while trying to learn French, one of the clearest metaphors occurs in the second paragraph. In this paragraph's last sentence, Sedaris writes:

"As an added discomfort, [the other students] were all young, attractive, and well-dressed, causing me to feel not unlike Pa Kettle trapped backstage after a fashion show."

While this metaphor might not mention language, that comes later, the term "Pa Kettle" refers to a 1940s hillbilly character that moves into a modern home after winning a prize. It refers to the feelings of being out of place. This idea of being out of place is the central idea in "Me Talk Pretty One Day."

While the verbally abusive French teacher provides much of the comedy in this essay, she is just the embodiment of how Sedaris feels in this class. In fact, he makes clear that this is how he feels anywhere in France where he might have to speak. He says his "fear and discomfort" accompanied him beyond the classroom room and "out onto the wide boulevards." 

According to this essay, speaking a foreign language in a foreign land is frightening. This is why the Pa Kettle metaphor is relevant. Sedaris feels like he's been transplanted from a place where he feels comfortable, learning French in America, to a place where he feels completely out of place, learning French in France.