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As fossil fuels become scarcer and their effects take a greater toll on the environment, the need for a renewable less polluting energy source is being felt. One of the most obvious and abundant answers is using a combination of water and gravity to create electricity. This harnessing of falling water has been used for around 2500 years, ever since the Ancient Greeks did it. Obviously, the Greeks did not use the water the to create electricity. They had no turbines, generators or power lines. These days, we do have those things. The basic idea behind a hydroelectric dam is to stop water and limit the flow of a river. This is accomplished more or less by a big wall. The wall contains a hole in the middle, called an intake hole or valve. The intake is regulated by a control gate. The gate can be raised or lowered to allow more or less water through. The amount of water directly influences the amount of force that spins the turbine that waits at the bottom of the penstock, which is basically a tube. After the spinning, the water keeps moving through the outflow, which is on the other side of the dam. The water then becomes the rest of the river, and continues flowing downstream. When the water spins the turbine, o