its not silly question..............i google some answers,

http://www.howstuffworks.com/question35.htm

At sea level, the boiling temperature for water is 212 degrees F or 100 degrees C. As soon as all of the liquid water has evaporated (or, in the case of the rice cooker, as soon as all of the water is absorbed by the rice), the temperature inside the container immediately rises. The appliance has a thermostat that can detect when the temperature rises above 212 degrees F in the container, and it turns itself off.

http://automaticricecooker.neurondev.com/zojirushi-rice-cooker/how-does-a-rice-cooker-work

A rice cooker contains a rice pan (a container) to hold the rice and water. A heating element to heat the food, and a lid with a small hole to release excess steam. A small temperature sensor is hidden in the bottom of the rice cooker. When the water in the rice pan begins to boil, the temperature of the food stabilizes at 212 degrees Fahrenheit. As the water boils, the rice absorbs the steam and some steam escapes through the lid. When the water is nearly all gone, the temperature of the food will begin to rise above 212 degrees. This change in temperature is monitored by the temperature sensor inside the rice cooker and triggers a switch to turn the cooker off.

Some rice cookers like neuro fuzzy logic rice cookers (a popular example is the zojirushi brand) are more sophisticated and will adjust the amount of heat based on the amount of water, and the amount of heat absorbed by the food. If too much water has been added to the rice pan, these rice cookers can adjust the heat to compensate. Most rice cookers however, do not have this feature.