Simile

A simile compares two unlike things using 'like' or 'as', so the comparison is offered openly rather than asserted, and the reader holds both images at once.

The paper was as thin as a moth wing. Two things stay two things, joined by as, and the reader keeps both in view. That is the difference from a metaphor, which would simply declare the paper a moth wing.

Because it is openly a comparison, a simile is the safer of the two. It is also the easier one to waste, and the waste always happens in the second half.

• Her hands were shaking like leaves. — the comparison is pre-owned; the reader learns nothing.
• Her hands were shaking like someone still holding a phone that had already gone dead. — the comparison carries information the sentence did not otherwise have.

A simile earns its place when the second term adds a fact, a feeling, or a piece of characterisation. When it only restates the first term more decoratively, it is purple prose with a like in it.

Three practical tests. Does the comparison point at something the reader could not already see? Is it drawn from a world this narrator actually knows? And is it the only simile in the paragraph — because two competing comparisons in three sentences will cancel each other out, and the reader will remember neither.