According to Piaget, there are four stages of child development that are broken up by age. A typical 8-18 month old, according to Piaget, would fall into the first category, which is the sensory motor stage. This stage covers birth to 2 years old.
During this stage, behavior is considered "ego-centric," meaning babies in this stage are largely unaware of others, and behaviors are controlled by personal needs and wants. Generally speaking, this stage is characterized by exploration through the senses. Babies explore their environments and the objects within them, first through their ears and mouths, then through eyes and hands. It is important to note that there is a huge difference in cognition between 8 months and 18 months. A typical 8 month old more than recognizes the voices most familiar to him. He very likely also recognizes his own name, though he cannot verbalize it. By 18 months, he is imitating and verbalizing at least 10 words, but likely more. Likewise, a typical 8 month old is still at the stage of putting absolutely everything he can reach directly in his mouth. By 18 months, though exploration by mouth is still normal, it is not as common, nor is it typically the first mode of discovery.
Problem solving is done by trial and error. For example, this is right around the age when babies discover that when an object is dropped, it falls to the floor. When mommy or daddy continually bends down to retrieve the missing object, the baby very often becomes delighted in the repetition of the behavior and (unknowingly) develops his or her sense of control. At 8 months, when an object is taken away, the baby believes it is gone completely. By 18 months, the baby has learned to look for the missing object.
There is certainly some debate as to the use of "punishment" and "rewards" at the 8 month mark with a baby, but some would suggest that this is not too early to begin introducing boundaries. Most experts agree, however, that it is sometime within the 8 to 18 month mark that children can associate very simple cause and effect. For example, a crawling child within this age range approaches a brick fire place, and mom or dad sternly says "No," and physically removes the child from the area. During the sensory motor stage, the voice inflection and physical movement has the ability to change the child's behavior, despite the fact that the child is far too young to understand the reasons for the boundary.
Similarly, children within this age range have, on the whole, learned that crying and closeness in physical proximity is the quickest way to alert someone of a need. Problem solving is most often accomplished through the repetition of behaviors which most often reward the child with what he or she wants. Though many doctors suggest that 18 months is too young for "temper tantrums," many mothers would disagree.
Most importantly, according to Piaget, the sensory motor stage largely does not include any cognition of the "why" behind behaviors. There is no ability to reason nor to understand the complex connectedness of things. There is comfort and security in routine and consistency. Unanticipated change, at this stage, very often causes an innate sense of insecurity.