The immutable law of shampoo

The container empties rapidly, but only until the last half inch or so remains. Then, the residue will last for months and months. The secret? Merely keep adding water, and the substance will keep lathering and cleaning. It's sort of like the story of Baucis and Philemon (this is the story as I first knew it, courtesy of Hawthorne). The pouring never ends. As to the shampoo, the greatest mystery is that the greatest portion goes so fast and that the remainder lasts for an eternity, or what passes for it these days.