They start to throw things. By the time they are 12 months old, most babies weigh three times what they weighed at birth and are two times as long as they were at birth. They cry to express hunger, anger, and pain. It is their way of communicating. They are easily excited or upset.
Infants smile in response to a pleasant sound or a full stomach.
At about 6 weeks, they smile in response to someone else. By 4 months, they smile broadly and laugh when pleased. They begin to fear being left by their parents or other care providers. They put anything they can hold into their mouth. By 12 months, many infants speak their first understandable words.