The simile to use is "as cool as a cucumber." Similes compare two different things using the words "as" or "like," and in this expression, the coolness of a cucumber is used to describe someone who remains calm and unflustered. Here's how it works: when someone is described as "as cool as a cucumber," it means they are composed and unflappable, even in stressful situations. This simile is common because cucumbers, even when the external temperature is high, tend to be cooler inside.
Expression
Meaning
As cool as a cucumber
Very calm and composed
This simile is widely recognized and used in everyday English language for its vivid imagery and aptness.