Both of the examples you provided contain elements of irony, but they illustrate different types: situational irony and dramatic irony.
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Brown Girl Dreaming: This example is primarily dramatic irony. The audience (or readers) knows that Jacqueline will grow up to be an author and will indeed write about her family, even though her mother believes that her daughter won't write about them. The irony lies in the contrast between the mother's expectation and the eventual reality that the reader is aware of but the characters are not.
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The Gift of the Magi: This example is situational irony. Della and Jim each make a sacrifice, selling their most prized possessions to buy gifts for each other. The situation is ironic because their sacrifices render the gifts useless—the watch chain can't be used without the watch, and the brush can't be used without the hair. The outcome is unexpected based on their intentions, which creates the irony.
In summary:
- Dramatic Irony in Brown Girl Dreaming
- Situational Irony in The Gift of the Magi