First Spoken Words
Piaget says that a babies first words are formed into categories that are built on sensorimotor foundations. The first 10 words of U.S. and Chinese babies were tracked and they fell into the categories of; common objects, important people, and sound effects. They were able to say things such as ball, mommy and daddy, and animal or car noises. The next most common set of words came from the categories of action words like hit or hug and then social routines like hi and bye.
Underextension
This is a problem that is shown when children learn their first words and they apply them too narrowly.
Overextension
Overextention is commonly made as an infants' vocabulary gets larger, and it is that they apply one word to cover a larger group of words. For example, a child will use the word car, to talk about all vehicles in general. This mistake is generally made because children have not learned or can not recall a certain word that would help them classify an object.
Berk, L. E. (2012). Infants and Children: Prenatal Through Middle Childhood. Boston: Pearson.