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.
Like oil and water.
This is used to describe people or situations that are incompatible and refers to the fact that oil and water have a natural tendency to separate.
E.g. They just can’t work well together in the team. They are like oil and water and just can’t seem get along with each other.
To drop like flies.
This is used to describe a situation where people are adversely affected by a problem or illness, usually in large numbers.
E.g. We are short-staffed this week and a lot of people are off ill. It’s the flu season and people are dropping like flies!
Have fun learning English!
Louise