If you brag that "the world's your oyster," you're using a metaphor from Shakespeare, who knew a thing or two about figures of speech.
Good writers know their way around a metaphor, where you make an analogy between two things to show how one resembles the other in some way. When a character from Shakespeare calls the world his oyster, that's his boastful way of saying that all the riches of the world are his for the taking, like plucking a pearl from an oyster shell. Shakespeare also wrote, "All the world's a stage." Oyster? Stage? Come on, Will, get your metaphors straight!
n a figure of speech in which an expression is used to refer to something that it does not literally denote in order to suggest a similarity
dead metaphor, frozen metaphor
a metaphor that has occurred so often that it has become a new meaning of the expression (e.g., `he is a snake' may once have been a metaphor but after years of use it has died and become a new sense of the word `snake')
a combination of two or more metaphors that together produce a ridiculous effect
a metaphor that exploits a similarity between experiences in different sense modalities