People's bathing habits have varied greatly over the centuries and among varying civilizations. Ancient Egyptians bathed for religious purposes, as did the Hindus of India. The city of Mohenjo-daro in the Indus Valley had a number of baths, complete with drainage system, in 2500 b.c. The Cretan Palace of Knossos and some ancient Greek palaces had separate bathrooms with individual tubs, piped-in water, and drainage systems. The Greeks considered bathing as a way to tone up the body, so they used cold water. The Romans turned bathing into a social occasion. Wealthy Romans had their own bathrooms. Soon enormous, elaborate public baths were constructed. These featured cold, warm, and hot bathing rooms, plus theaters libraries, exercise yards, and more. The Baths of Diocletian, built in a.d. 302, could hold 3,200 people at once; the Baths of Caracalla, built in 217, held 1,600 people. The Roman baths were...