Participants will undergo MRI scanning of the brain during rest or while performing a task designed to test a skill. The tasks may involve remembering numbers, reading a word, tapping fingers, or looking at a flashing picture. MRI uses a magnetic field and radio waves to produce pictures of the brain. For the procedure, the subject lies still on a table that is moved into the scanner (a cylinder containing the magnet). Earplugs are worn to muffle loud noises caused by electrical switching of radio frequency circuits used in the scanning process. Adults may spend as long as 90 minutes in the scanner, usually less than 45 minutes. For children, the time is less than 75 minutes-usually 30 to 40 minutes. Participants may be asked to repeat the scans up to 5 times in different sessions to test different brain functions or confirm findings.