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 useful as a chocolate teapot.
If someone or something is as useful as a chocolate teapot they are of no practical use at all. They are useless.
E.g. When there are no roads a car is about as useful as a chocolate teapot.
As white as a ghost.
If someone is as white as a ghost it means they are very pale in the face (possibly due to feeling frightened).
E.g. When she saw him pull out the knife she turned as white as a ghost and started to run.
Have fun learning English!
Louise