Spend time with the family of Bill Gates, and eventually someone will mention the water incident. The future software mogul was a headstrong 12-year-old and was having a particularly nasty argument with his mother at the dinner table. Fed up, his father threw a glass of cold water in the boy's face. "Thanks for the shower," the young Mr. Gates snapped. The incident lives in Gates family lore not just for its drama but also because it was a rare time that Bill Gates Sr., father of his famous namesake, lost his cool. The argument presaged a turning point in the life of a tempestuous boy that would set him on course to become the Bill Gates whom the public knows as co-founder of Microsoft Corp. and the world's richest man. Behind the Bill Gates success story is the other William Gates. The senior Mr. Gates balanced a family thrown off kilter by a boy who appeared to gain the intellect of an adult almost overnight. He served as a quiet counsel as his son jumped into and thrived in the cutthroat business world. When huge wealth put new pressure on the son, the elder Gates stepped in to start what is now the world's largest private philanthropy.