Final answer:
Tides are caused by the Moon's gravitational pull on Earth, resulting in two high tides per day.
Step-by-step explanation:
Tides are caused by the Moon's gravitational pull on Earth. The Moon pulls more strongly on the side of Earth closest to it, creating a high tide. However, Earth is also pulled towards the Moon more than the water on the far side, resulting in a second high tide on the opposite side of Earth. As Earth rotates, the tidal bulge keeps its orientation with the Moon, resulting in two tides per day. The actual tidal period is about 12 hours and 25.2 minutes.