Thats why we call her mommy, is a story about why someone is called Mommy. It describes how and why it is a term of endearment rather than a title. To have a mommy, doesnt only mean having a birth mom. It can include foster moms, adoptive moms, step moms, etc.. It teaches that as long as someone loves you unconditionally and takes care of you the way a mother should, they can be called a Mommy.