Children under the age of three do not understand "no" in the way most parents think they do. (And, a full understanding of "no" doesn't occur magically when the child turns three. It is a developmental process.) "No" is an abstract concept that is in direct opposition to the developmental need of young children to explore their world and to develop their sense of autonomy and initiative, as discussed in Chapters 4 and 5 of Positive Discipline for Preschoolers.