Why is the idli batter left undisturbed overnight at a warm place ? What do we observe when we look batter in the morning ?
The idli batter when kept overnight undisturbed rises up.
This happen due to the process of fermentation. As yeasts belong to a group of simple organisms known as fungi, which exist almost everywhere in nature, including the air. Yeasts reproduce rapidly, and they grow especially well in substances containing sugar (as in the idli batter).
Hope this answer clears your doubt.