We think about it, sing about it, dream about it,lose sleep worrying about it. When we don't have it; we search for it; when we discover it; we don't know what to do with it; when we have it; we fear losing it. It is the constant source of pleasure and pain. But we can't predict which it will be from one moment to the next. It is a short word, easy to spell, difficult to define, impossible to live without.