Turn any soap bar, whether homemade or store-bought, into liquid soap.
Bring the 3 cups of water to a boil.
Grate the soap or finely chop. Fill the grated soap into a quart-sized heat-resistant bowl or jar.
Pour the boiling water over the grated soap and stir to combine. Once fully combined cover and let sit for 45 minutes. Stirring occasionally.
The soap mixture should be slightly thickened by now. Add the glycerin if desired (see note).
Fill the soap mixture into a quart-sized mason jar (if you didn't make it in one) and use as desired.
Note: The glycerin gives the soap a smoother texture and prevents it from getting very gel-like.
If the soap is too thick after 45 min, heat some more water and add it to the mixture, stirring to combine.
If too liquid, transfer to a pot and heat on the stove top adding more grated soap. Should the soap separate after being stored, simply shake to mix again.