The level of the sea rises and falls about twice every day. We call these changes tides.
Tides are caused by the difference in gravity across the width of a planet.
The tides 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 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+)