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Physical theories with extra dimensions in addition to the three spatial dimensions and one time dimension we appear to live in may explain the weakness of gravity relative to the other forces of nature. But why haven't we observed these extra dimensions directly? An extra dimension can curl up in a manner that is nearly impossible to discern for an inhabitant of the larger, uncurled dimensions. For example, a two-dimensional sheet appears nearly one-dimensional if it is rolled into a tight cylinder. If all of the forces can be felt in the extra dimensions, then the curled up dimensions must be smaller than 2x10-17 cm. If, however, only gravity penetrates the extra dimension, it can be as large as a tenth of a millimeter. (Unit: 2)