"Alton Brown from the Food Network used the analogy of a train car. An unsaturated fat is like a car with lots of empty seats. A saturated fat is like a car with all the seats filled. A trans fat has most of the seats filled, usually with hydrogen, hence the term "partially hydrogenated". An unsaturated fat can collect unsavory passengers, like sulphur molecules, which impart that rancid flavor that everyone hates. Trans fats avoid collecting passengers, dont get rancid, so the food processors love them."