My child's poop smells better than your child's

A study shows that mom's prefer the smell their own child's poop to that of other children's poop. One theory is that moms evolved to this so that they will care for their children.

Frankly, as a father, I knew that I had to change my child's diaper but I didn't have to do so for another person. My daughters babysit and they change diapers without complaint. But this isn't quite the same. It's one thing to change a diaper, it's another to smell different diapers (without having to change them) and prefer one smell over another.

A mystery?