To use it, a dam is built to trap water, usually in a valley where there is an existing lake. Water is allowed to flow through tunnels called penstocks in the dam, to turn turbines and then drive generators. The dam has to be most thick at the bottom because the pressure of the water is so much greater. (See pressure in liquids) This method is possibly the most reliable way of making renewable energy. Also, unlike solar, which only works when it is sunny, or wind, which needs wind, hydro electric works throughout the year, unless there is a drought.