What is soap? Soap is traditionally made by adding lye to fat or oil – a process called saponification. This results in salts of fatty acids which clean by removing the fat that surrounds dirt on our skin, thus enabling water to rinse it off. A good soap meets 2 criteria: a) it is made from good quality oil of plant origin and b) the oil content is so high that not all of it is saponified, leaving excess to ...