A psychological experiment starts with the formulation of a problem, which is usually best stated in the form of a question. The only requirement that the problem must meet is that it be solvable — the question that it raises must be answerable with the tools that are available to the psychologist. Psychological testing is a field characterized by the use of samples of behaviour in order to assess psychological construct(s), such as cognitive and emotional functioning, about a given individual. Learning is acquiring new knowledge, behaviours, skills, values, preferences or understanding, and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.