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 light as a feather.
If something is as light as a feather it is extremely light and doesn’t weigh much.
E.g. You can pick this up easily – it’s as light as a feather.
As bold as brass.
If someone is as bold as brass they are extremely confident, brazen and can be perceived as being rude or impolite.
E.g. She walked up to the counter, as bold as brass, and demanded her money back, saying she had been treated poorly.
Have fun learning English!
Louise