Explain in short :
Tidal energy

Explanation

Tidal energy : The level of water in the sea rises and falls mainly due to the gravitational pull of the moon on the spinning earth.
The phenomenon of the changes the sea level is called high and low tides.
The difference in sea level gives tidal energy which is harnessed by constructing a dam across a narrow opening to the sea. A turbine fixed at the opening of the dam converts tidal energy to electricity.