Hydroelectricity is energy generated by water. We use the energy of water falling from a higher place to a lower place (like a river flowing downhill) and we turn this energy into electricity. First, we build a big dam across the river to store the water and create a big lake. Then we build a powerhouse next to the dam, which has a turbine inside that spins around when water flows through it. The spinning turbine generates electricity, which flows through power lines to people's homes.