Robert Chapman, age 13, of Redkey, Indiana, for his question:

What makes rain water so soft?

The softest water is pure, distilled water with no dissolved chemicals or particles of any sort. In nature, the nearest thing to this is the cloudy moisture that condenses from water vapor in the air. These droplets congeal together to form rain¬drops, heavy enough to fall. Water, however, is a great solvent, always eager to dissolve surrounding particles. In polluted regions, the clouds contain a large assortment of floating particles. To same degree, they contaminate the softness of the falling rain.

Even in pollution free regions, raindrops absorb carbon dioxide as they fall through the air. But the runoff remains soft. Hardness occurs when rain sinks through the ground, especially in regions of limestone and other soluble rocks. These chemicals are dissolved and added to the water. Sometimes they also add a mineral flavor  ¬and always they oppose the frothy chemical action of soaps.