Final answer:
The Moon's gravity causes tides on Earth. There are two high tides per day due to the alignment of the tidal bulge with the Moon's position.
Step-by-step explanation:
The Moon's gravity causes tides on Earth. The Moon exerts a stronger gravitational force on the side of Earth closest to it, creating a high tide. On the opposite side of Earth, the Moon's gravitational pull is weaker, resulting in another high tide. As Earth rotates, the tidal bulge stays aligned with the Moon, causing two high tides and two low tides per day.