Memories are classified as short-term or long-term. Short-term memories, which sometimes last only a few seconds, are usually concerned with material of fleeting importance such as telephone numbers. Long-term memories, which may last as long as a lifetime, are concerned with material of great significance or possible future use.
Memories may be visual or verbal in content and reproduction. Verbal material includes ideas as well as poems, word lists, dates, and names. Memories acquired through motor activities, and which evoke motor responses—such as the ability to swim—are called skills. Emotional effects may also be stored and reproduced. A child frightened by a dog may retain the fear for many years. It was once thought that skills and emotional memories were retained longer than verbal memories, but experiments have not upheld this theory.