String theorists generally believe that extra dimensions are compactified, or curled up, so that the ten-dimensional spacetime required for string theory to combine gravity with the other forces of nature appears to be four-dimensional. A simple example of compactification leading to an apparently smaller number of dimensions is a flat sheet rolled into a tube. The flat sheet is two-dimensional, but when rolled into a cylinder with a very small radius, appears to be a one-dimensional line. (Unit: 4)