Using one thing to represent something else is metaphor. Sometimes a metaphor helps your audience understand a concept better, or see it in a new way. Once you have found a metaphor for your message, take the opportunity to explore it fully. Go beyond clichés for real creativity. Don’t mix metaphors!
Cliché: He weathered the storm of scrutiny.
Mixed: She kept her nose to the grindstone until all the pieces fit together.
Creative: As thoughts ricocheted between neurons, his eyes lit up with the 1000 point bonus idea.
An example from the master:
“I think our country sinks beneath the yoke;
It weeps, it bleeds, and each new day a gash
Is added to her wounds.”
—Macduff to Malcolm, in Macbeth