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 smooth as silk.
If something is as smooth as silk it feels very soft.
E.g. My skins always feels as smooth as silk after using that cream.
It can also be used to describe a process or event that ran without problems, obstacles or hindrances.
E.g. The negotiations were as smooth as silk. The deal is done!
It can also be used to describe a person who appears to be very polite and confident but is often not sincere in what they say or do.
E.g. Be careful dealing with him. He’s as smooth as silk.
Have fun learning English!
Louise