A simile is usually preceded by the word "like" or "as" or some equivalent. In Act 1, Scene 7 of Macbeth there is a famous simile which is intriguing for several reasons, including its extravagance. It comes right in the opening soliloquy of Macbeth, which begins with, "If it were done when 'tis done, then 'twere well / It were done quickly." The simile comes toward the end of that soliloquy:
And pity, like a naked newborn babe
Striding the blast, or heaven's cherubin horsed
Upon the sightless couriers of the air,
Shall blow the horrid deed in every eye,
That tears shall drown the wind.
If you find that one a little hard to figure out, you're not alone. Scholars have been analyzing it for centuries. I suppose one would feel pity for a naked infant exposed to a windstorm. The cherubin were angels, and in Macbeth's simile the couriers were not blind but invisible horses evidently coming to exact revenge for the murder. The word "horsed" shows that the "sightless couriers" are invisible horses. Essentially all Macbeth is saying is that everybody in the kingdom will feel great pity for Duncan because he is loved. The word "pity" is the key word.
If you would like a much simpler simile, Lady Macbeth shortly later askes her husband:
Wouldst thou have that
Which thou esteem'st the ornament of life
And live a coward in thine own esteem,
Letting "I dare not" wait upon "I would,"
Like the poor cat i' th' adage? (1.7.45-49)
Evidently the adage, or proverb, had to do with a cat that wanted to catch a fish in a pool of water but was afraid of getting drowned.
I suspect that the teacher asked for a simile in Act 1, Scene 7, because the teacher wanted to see how many students would catch the one about the naked newborn babe striding the blast. If I were answering the question in your class, I would use that one, even though it might be a little hard to explain.