As their gut becomes colonized with bacteria, the stool becomes smellier. This is a normal process. Breast-fed babies usually don't have very smelly stools, while formula-fed babies often have a more pungent odor. Generally speaking, poop stinks, and there's not much we can do about it. The scent is most often a reflection of how long the poop was in the intestines—the longer it sits in bacteria, the more it'll smell. However, some babies with very sour- or foul-smelling poop may have an intolerance or allergy.