the sea level rises and falls at regular intervals during the course of a day.such regular periodic changes in the sea level are called tides.when the sea level rises, it is known as high tide and when the sea level falls ,it is called low tide.tides are helpful for navigation.ships can enter or leave the harbour easily during high tide , owing to the greater depth of water.tidal energy is now being used to generate power.gravitational attraction f the moon and the sun causes tide.
