Learning is the process of acquiring new understanding, knowledge, behaviors, skills, values, attitudes, and preferences. The ability to learn is possessed by humans, animals, and some machines; there is also evidence for some kind of learning in certain plants. Some learning is immediate, induced by a single event, but much skill and knowledge accumulate from repeated experiences. The changes induced by learning often last a lifetime, and it is hard to distinguish learned material that seems to be "lost" from that which cannot be retrieved. Human learning starts at birth and continues until death as a consequence of ongoing interactions between people and their environment. The nature and processes involved in learning are studied in many established fields, as well as emerging fields of knowledge. Research in such fields has led to the identification of various sorts of learning.