In the study of problem solving, any problem in which the initial state or starting position, the allowable operations, and the goal state are clearly specified, and a unique solution can be shown to exist. Typical examples are the Tower of Hanoi, Wason selection task, and water-jar problems. A problem that lacks one or more of these specified properties is an ill-defined problem, and most problems that are encountered in everyday life fall into this category. Also called a well-structured problem. See also nine-dot problem, problem-solving stages.