French

"I could invent a teakettle that reads in Dad’s voice, so I could fall asleep, or maybe a set of kettles that sings the chorus of 'Yellow Submarine,' which is a song by the Beatles, who I love, because entomology is one of my raisons d’être, which is a French expression that I know." (1)
Showing his air of superiority because he is a child and feels he is smarter than everyone else
"That made me feel self-conscious. 'Excusez-moi?' I told him" (2).

"Then I thought of 'porte', which is French for door, obviously" (10).

"raisons d'etre" (1 and 7)

"Another good thing is that I could train my anus to talk when I farted [...] and if I ever made an incredibly bad fart in the Hall of Mirrors, which is in Versailles, which is outside of Paris, which is in France, obviously, my anus would say, "Ce n'étais pas moi!" (1)

This quote comes from just after his father finishes the story of the six boroughs:
"When the story finished , we turned on the radio back on and found someone speaking French. That was especially nice, because it reminded me of the vacation we just came back from, which I wish never ended." (14)

"'I said I kicked a French chicken in the stomach once.' 'Huh?' 'It said, "Oeuf"' 'What is that?' 'It's a joke. Do you want to hear another or have you already had un oeuf?'" (5-6)

"Are you sure?" "Tres fatigue," I said waving my hand." (168)

"I know he just wanted to be friendly but it made me incredibly angry. "Yo-yo moi!" I told him, grabbing it back. " (3)
I thought this was interesting because maybe it's his way of expressing himself, relating to his father.
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Actually, this one is doubly interesting, because he gets it wrong. "Yo-yo moi!" really means "yo-yo me", rather than "yo-yo mine", which was what he probably meant to say. Seeing as he's just spent the last couple of pages inadvertently boasting about how clever he is (explaining what a pacifist was just in case you didn't know; bragging about how much French he knows etc), this mistake brings him down a few notches in the reader's eyes from being the smart, funny and slightly peculiar kid to any other small boy who's a bit too big for his boots. At least, it did for me.
Also, "yo-yo moi" is funny because it sounds a bit like Yo Yo Ma. Just gonna throw that in there.

It is also interesting to note that "Yo-Yo moi" would be considered appropriate for a casual possesive (chez moi, chez toi, etc.) in French conversation, which probably means that Oskar has studied the language and its usage more than the average American student.

I think that this boy uses French all the time, is because it is his only link to remembering his father, and i think it helps him get past the sadness of his death.

I think Oskar is kind of a show off and really likes to make himself seem as intelligent as possible, so maybe this is a reason he slides little tid bits of french language in is everyday speech. However, Oskar does seem to slow down on his french vocabulary as the book progresses, why?

Raisons D'étre is a french phrase...

I think it is interesting that his mom ended his french lessons

oskars dad

"The very same engineers who dealt with the Leaning Tower of Pisa...which was where?" "ITALY!" "Right..." (219)
Oskar's dad telling Oskar about the sixth Borough