A cis isomer is a molecule in which two identical or similar groups sit on the same side of a rigid bond, typically a carbon-carbon double bond. The arrangement shapes the molecule’s geometry and properties: cis unsaturated fatty acids, the natural form in most foods, have a kinked structure keeping fats fluid, while trans isomers, with groups on opposite sides, pack straight and behave more like saturated fats. Partial hydrogenation created trans fats with adverse health effects, now largely removed from food supplies. The cis-trans distinction shows how a subtle geometric difference transforms a fat’s behaviour and health impact.