Mexican soap, or saponaria, is a traditional form of soap made from the saponaria plant. It has a light, refreshing scent and is gentle on the skin. Mexican soap is traditionally used to cleanse the body and hair, but it can also be used for laundry and dishes. To use Mexican soap, wet the bar and rub it between your hands to create a lather. Apply the lather to your body or surface and rinse with water.