oil and water
Two objects, elements, factors, forces, people, etc., that do not or cannot mix together readily. (Usually used comparatively, as in "X and Y are like/as oil and water.") Refers to the natural tendency of oil and water to separate. My dad is like oil and water with my mom's boyfriend, so I don't know how they're going to get through the family reunion in each other's company. The more I pay attention to the news, the more it seems that politics and common sense are as compatible as oil and water.