The document discusses how the moon's gravitational pull causes Earth's tides. It explains that the moon, being closer to Earth than the sun, has a stronger tidal influence. The moon's gravity causes the oceans to bulge on both the near and far sides of Earth, resulting in two high tides per day. It also describes spring tides with extra high and low tides during new and full moons, and neap tides with less pronounced tides during half moons due to the sun partially offsetting the moon's gravitational pull.