Idiom: “when fish fly”
Meaning: Something that will never 
happen.
Example:
Veronica: I’ll buy a pink dress tomorrow.
Elena: Yeah, right! You’ll buy a pink dress when fish fly!
Idiom: “money pit”
Meaning: A business, a project or a possession (a house, a car) that
requires more and more money with little hope of profit. 
Example:
Veronica: See this new car? It’s a money pit! I have to take it to the mechanic every week….
Elena: Why don’t you get rid of it and take the subway like everybody else?
Idiom: “a fat cat”
Meaning: A very rich and powerful person
Example:
Veronica: Look at that guy’s car! It’s a Ferrari!
Elena: He’s a fat cat for sure.