How milk is changed into curd, if a few drops of curd is added to it? What is the reason for its sourness?

How milk is changed into curd, if a few drops of curd is added to it?

1.	How milk is changed into curd, if a few drops of curd is added to it? What is the reason for its sourness?
Answer» The change is brought by Lactobacillus lactis, a bacterium present in the curd. The sourness is due to the formation of lactic acid.