There are many expressions commonly used in English with the comparison form ‘as … as’.
There is no magic rule to help you remember them. You just have to learn them. Sorry!
Take a look at the examples below.
As fit as a fiddle.
If someone is as fit as a fiddle they are in an excellent state of health.
E.g. Although my grandmother is over 90 she’s as fit as a fiddle.
As sly as a fox.
If someone is as sly as fox it means they are clever at getting what they want, often in an underhand or manipulative way.
E.g. Be wary at the meeting. He’s an experienced salesman and as sly as a fox. Don’t sign something you are not sure of!
As cool as a cucumber.
If someone is as cool as a cucumber they are not easily stressed by situations and can remain calm under pressure.
E.g. He’s an excellent negotiator. Even when the pressure is on to get a good deal he remains as cool as a cucumber throughout.
Have fun learning English!
Louise