Why idli batter rises when kept over night?

Idli batter rises overnight due to the fermentation process facilitated by naturally occurring yeast and lactic acid bacteria in the batter. As these microorganisms consume the carbohydrates in the rice and lentils, they produce carbon dioxide gas, which causes the batter to expand and become airy. The warm temperature also accelerates fermentation, resulting in a light and fluffy texture ideal for making idlis. Additionally, the production of lactic acid contributes to the characteristic tangy flavor of fermented foods.