I live in a hard water area. The calcium and magnesium present in my water will cause soap scum. Soap makes grease soluble in water by latching onto it with the fatty acid end. The sodium end is soluble in water and will pull the greasy part down the drain. A basic soap molecule looks something like this, to put it very crudly: (fatty acid...............sodium). The fatty acid end is attracted to grease and the sodium end is attracted to water (I do have a point...) SO, in the presence of hard water, the calcium or magnesium will replace this sodium end, thus making the entire soap molecule INsoluble in water: soap scum. Use of distilled water will ensure that you are making a nice bar of soap, not soap scum. Hope this makes sense...