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 clear as mud.
If something is as clear as mud it’s very difficult (or impossible) to see or understand.
E.g. I have no idea what I’m doing. The instructions in the user manual are as clear as mud!
As easy as shooting fish in a barrel.
If something is as easy as shooting fish in a barrel it is exceptionally easy to accomplish or achieve.
E.g. He does this every day. It’s as easy as shooting fish in a barrel for him.
Have fun learning English!
Louise