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 flat as a pancake.
If you say that something is as flat as a pancake you are emphasizing that the object / thing is extremely flat.
E.g. Certain areas of the country have hills and mountains and other areas are as flat as a pancake.
As free as a bird.
If someone is as free as a bird they have complete freedom to do exactly what they want when they want to.
E.g. Since she retired from her job she’s been as free as a bird to do exactly what she wants.
As hard as nails.
If someone is as hard as nails it means that they don’t show emotion, sympathy or pity towards other people. It has the sense that the person is uncaring towards others.
E.g. Don’t expect any sympathy for your situation from her. She’s as hard as nails and probably doesn’t care.
Have fun learning English!
Louise