When lentils are prepared, they are first inspected for damaged lentils, stones and other foreign matter. Then they are rinsed until the water runs through and comes out clear. Some prefer to soak the lentils for a longer time and discard the water. This removes substances that may cause indigestion. The lentils are then boiled in water or broth. They may be cooked on the stovetop, or in a slow cooker. Pressure cookers are not recommended, since the small lentils may clog the pressure relief valve, and their quick cooking time means there is little benefit from pressure cooking. Cooked lentils often require thinning: adding more hot water or broth to the cooked legumes until the desired final consistency is reached.