There are many expressions commonly used in English to express similarity.
There is no magic rule to help you remember them. You just have to learn them. Sorry!
Take a look at the examples below.
(To be) like water off a duck’s back.
If something is like water off a duck’s back to a person, it means it has no impact on that person. It doesn’t affect that person at all. This phrase is often used with regard to insults or criticisms.
E.g. She told him he was lazy but he didn’t care – it was like water off a duck’s back to him.
(To be) like gold dust.
If something is like gold dust, it is very difficult to find or obtain. It may also be valuable if a lot of people are trying to find or obtain it.
E.g. I’ve tried and tried but I cannot get tickets for the concert. They are like gold dust.
Have fun learning English!
Louise