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 bright as a button.
If someone is as bright as a button they are intelligent, a quick-thinker, lively and alert.
E.g. Don’t worry, he’ll find a solution to your problem. He’s as bright as a button.
As common as muck.
If you describe someone as being as common as muck, you believe they come from a low social class or have low social status. It is most often heard in the U.K. It’s informal language and potentially offensive.
E.g. Look at what she’s wearing! It’s totally inappropriate for this setting. She’s as common as muck!
Have fun learning English!
Louise