This book introduces the Five-hundred-meter Aperture Spherical Radio Telescope (FAST), also known as the "The Sky Eye", one of the China's big science facilities. Using easy-to-understand language, the book covers the scientific background, construction process, achievements, and effects of FAST on the development of the national economy. With a large number of intuitive and vivid images and videos, the book presents basic information on FAST for general readers who are interested in astronomy.