What causes oil and water to never mix together?

Oil and water don't mix because of their molecular structures and the forces between them. Water molecules are polar, meaning they have a slight positive charge on one end and negative on the other, which causes them to form strong hydrogen bonds with each other. Oil molecules are nonpolar, with no such charge difference. When oil is introduced to water, the water molecules are more attracted to each other than to the oil molecules, so they effectively push the oil out and maintain their hydrogen bonds. This is summarized by the chemistry principle "like dissolves like," meaning polar substances mix with polar substances and nonpolar with nonpolar. Since oil and water are opposites in this regard, they remain separated.