Fifteen years ago, textbooks on human development stated that babies of six months of age or younger had no sense of "object permanence" -- the psychological term that describes an infant's belief that an object still exists even when it is out of sight. That meant that if mom or dad wasn't in the same room with junior, junior didn't have the sense that his parents were still in the world. These days, psychologists know that isn't true: for young babies, out of sight doesn't automatically mean out of mind. But how much do babies remember about the world around them, and what details do their brains need to absorb in order to help them keep track of those things? Babies may not remember what they saw, but they remember that they saw something.