Question

Why does the dough prepared for making idlis rises when kept overnight ?

Answer 1

becoz in dough there would be yeast. which while reproduce release CO2. so the dough increase over night due to which co2 pushed it forward

same principle in bread making too

Answer 2

This happens due to the process of fermentation which has taken overnight. And during this process of fermentation, a large amount of CO2 is produced which result in raising of dough.