A detective is showing off notes to the lieutenant of the precinct he works at. Being a more classy investigator, he keeps a journal of notes that he separates by case. However, in order to ensure that no one copies and/or plagiarizes his work, he takes the notes in a different way.
Because he is a fan of linguistic riddles, he writes his entries down in a way that can only make sense through thinking about what the entry is truly saying. In other words, it looks like nonsense to the average human.
He(the detective)was instructed with finding where a certain knife was hidden, as it was the murder weapon of the case being investigated. The entry showing how he deduced the location went like this.
La montagne russe
So... where is the knife hidden?