Question

why does a foul smell comes when wheat dough is left open for few days

Answer 1

because it comes in contact with the moisture present in polluted air

Answer 2

Wheat dough when kept in open for few days, following activities takes place:

 formation of fungus and . reproduction of yeast

These fungus and yeast when present in dough, it will become unfit for eating or for use. Since formation of fungus takes place in dough, so it gives a foul smell.