instruction: What causes ocean tides?
response: Tides are very long waves moving across the ocean and are caused by the gravitational forces exerted on the earth by the moon. When the highest point in the long-period wave reaches a shoreline the area experiences high tide. When the lowest point of the wave, the trough, reaches shore the area experiences low tide. The earth is mostly water and the water "points" at the moon, the parts of the Earth experiencing high tide is the water drawing towards the moon. The "pointing" water that faces the moon is formed because the gravitational pull of the moon is strongest on whichever side of the Earth faces it. Gravity pulls the ocean towards the moon and high tide occurs.
  
  The bulge on the far side of the Earth is caused by inertia. The water moving away from the moon resists the gravitational forces that attempt to pull it in the opposite direction. Because the gravitational pull of the moon is weaker on the far side of the Earth, inertia wins, the ocean bulges out and high tide occurs. As the Earth spins, different areas of the planet face the moon, and this rotation causes the tides to cycle around the planet.