Every now and then, the Sun, Earth, and Moon align so that, when viewed from the Earth, the Moon eclipses the Sun's light. Solar eclipses are fairly common -- the Moon will block out some portion of the Sun at least twice a year. However, it is still a special event to be able to witness a total solar eclipse. In this video segment adapted from NASA, learn how solar eclipses happen and why they are so difficult to witness. This video is available in both English and Spanish audio, along with corresponding closed captions.