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 dead a dodo.
If something is as dead as a dodo it is no longer alive, in existence, active, relevant or popular.
E.g. The last round of negotiations went really badly. At the moment the deal looks to be as dead as a dodo.
As different as chalk and cheese.
If two people are as different as chalk and cheese they are very, very different from one another. They are incompatible and may be in conflict.
E.g. His two daughters are like chalk and cheese. They dress very differently and have totally different hobbies and interests.
As dull as ditchwater.
If someone or something is as dull as ditchwater they are very boring, unexciting and just not very interesting.
E.g. The presentation was as dull as ditchwater. I nearly fell asleep.
Have fun learning English!
Louise