you can't make fish of one and fowl of the other

The phrase "you can't make fish of one and fowl of the other" means that it is impossible to combine or treat two things that are fundamentally different or incompatible as if they were the same. It suggests that it is not feasible to mix two distinct entities or try to change their nature to become similar to each other. The phrase often conveys the idea that some things just cannot be altered or blended because they have inherent differences.