An astronaut in training for an extra-vehicular activity mission using an underwater simulation environment. Training refers to the acquisition of knowledge, skills, and competencies as a result of the teaching of vocational or practical skills and knowledge that relates to specific useful skills. It forms the core of apprenticeships and provides the backbone of content at technical colleges and polytechnics. In addition to the basic training required for a trade, occupation or profession, it is recognised today that there is need to continue training beyond initial qualifications to maintain, upgrade and update skills throughout working life. In the context of many professions and occupations this may be referred to as professional development. A similar term used for workplace learning to improve performance is training and development. Such training can be generally categorized as on-the-job or off-the-job. On-the-job describes training that is given in a normal working situation, using the actual tools, equipment, documents or materials that they will use when fully trained. On-the-job training is usually most effective for vocational work. Off-the-job training takes place away from normal work situation which means that the employee is not regarded as productive worker when training is taking place. An advantage of off-the-job training is that it allows people to get away from work and totally concentrate on the training being given. This type of training is most effective for training concepts and ideas. Physical training is more mechanistic: training programs develop specific skills or muscles with a view to peaking at a particular time. A field of training often used in sports is autogenic training. Another type of training is fartlek training which is a flexible training type which can be adapted to suit almost any athlete. In military use, training means gaining the physical ability to perform and survive in combat, and learning the many skills needed in a time of war. These include how to use a variety of weapons, outdoor survival skills, and how to survive capture by the enemy, among others. See military education and training. Training methods have been developed for artificial intelligence as well. Evolutionary algorithms, including genetic programming and other methods of machine learning use a system of feedback based on "fitness functions" to allow computer programs to determine how well a task is being performed. A series of programs, known as a “population” of programs are automatically changed and then automatically tested for “fitness,” i.e. how well they perform the intended task. New programs are automatically generated based on members of the population that perform the best. These new members replace programs that perform the worst. The procedure is repeated until optimum performance is achieved.[1] In robotics, such a system can continue to run in real-time after initial training, allowing robots to adapt to new situations and changes in itself, for example due to wear or damage. Robots have also been developed that can appear to mimic simple human behavior as a starting point for training.[2] In religious and spiritual use, training means purifying mind, heart, understanding and actions to obtain a variety of spiritual goals such as closeness to God or freedom from suffering. Typical of institutionalized spiritual trainings is the Buddhist Threefold Training. Training differs from exercise in that exercise may be an occasional activity for fun. Training is specific and done to improve one's capability, capacity, and performance.