In "Oh, My Love is Like a Red, Red Rose," what is an example of a metaphor?
As the other answers to this question note, a metaphor in Robert Burns's poem occurs at the end of the third stanza in the "sands of life" line. I'd like to expand upon one of the implications of this metaphor. If "the sands of life" compares the span of our lives to the running sands of an hourglass (which it most certainly does), then there's an implication here that deserves recognition. By comparing our lives to the running sands of an hourglass, Burns very cleverly evokes the fact that, sooner or later, our time on earth will run out, just like the sand in an hourglass. Thus, no matter how passionate the speaker's love is for his beloved, his love will also end. The idea of temporary love makes sense within the greater context of the poem: if the speaker's love is like "a red, red rose," then this love is not only beautiful, but also fated to wither and die, just as roses do. Thus, if Burns's metaphors and similes are celebrating the beauty of his beloved and the strength of his love for her, then they're also referencing the fact that love is temporary and will end at some point. This interpretation adds a poignant, more melancholy air to the verse that isn't at first apparent.
Let us remember that a metaphor is different from a simile in that it asserts a comparison between an object and something else directly, without using the words "like" or "as." Of course, this poem is most famous for how the speaker's love is "like" a red rose, which is a simile, but if we look carefully we can see that there is an implied metaphor used in the third stanza:
Till all the seas go dry, my Dear,
And the rocks melt with the sun!
O I will love you still, my Dear,
While the sands of life shall run.
"The sands of life" is a metaphor as it compares life and the allotted span that each of us have to a sand timer, which is constantly allowing sand to run from the top chamber to the bottom chamber, indicating how much life we have left and also symbolising the life we have in the "running" of the sand. The metaphor thus reinforces the speaker's love for his beloved, by emphasising the way that his love will continue whilst he has breath in his body.