The level of the sea rises and falls about twice every day. We call these changes tides.
The tides on Earth are mainly caused by the gravitational pull of the Moon, but they are also changed a bit by the gravitational pull of the Sun. When the Sun and Moon are working together, the tides are particularly high, and are known as Spring tides. This happens at the full and new moon.
The Moon's gravity pulls the oceans towards it, however, on the other side of the Earth we still see a high tide - this is caused by the Earth's rotation. The spinning of the Earth produces something called "centrifugal force" which pulls the Earth, and the oceans upon it, away from the centre of mass between the Earth and Moon, which lies underneath the Earth's surface.
The following demonstrations help to explain why we have two tides every day, and why the timing of high and low tide changes with the orbit, or phase of the Moon.
Tides Demonstration (Age 11+)
Tides Simulator (Age 14+)