The answer by copycat is right. It has a thermostat and set to turn into warm when temperature reaches about 100 degree Celsius or more. When you cook rice the water evaporates and temperature reaches above 100 deg celsius and the thermostat turns it into warm mode. But when you boil only water, the temperature is still below or about 100 deg celsius as water boils at 100 deg celsius so does not go to warm mode. If you keep the cooker on then all water will evaporates and then it turns to warm mode, in principle. But in practice you may burn out your cooker as it takes a long time to evaporate all the water.