The overriding theme of the story is the death of innocence. In this particular case, Rosaura's loses her innocence as a result of Señora Ines's blatant social snobbery. Rosaura has gone to the party expecting to have fun with her friend Luciana and the other girls. But when she gets there she's disappointed to find herself being treated like a glorified servant.
While everyone else is enjoying themselves, Rosaura is helping to pass the cake around to the party guests. It's clear that Señora Ines doesn't regard Rosaura as a social equal; Rosaura's only at the party because Señora Ines wants to use her as some kind of waitress. When Señora Ines calls Rosaura her pet, it's a further sign of her patronizing disregard for a young girl she regards as her social inferior.
The whole experience is demoralizing and humiliating for Rosaura. Even more than that, it destroys her innocence. Far from having a fun time at the party, Rosaura receives a crash course in the adult world and all its...
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