Infants begin to understand words such as "Mommy", "Daddy", "hands" and "feet" when they are approximately 6 months old.[1][2] Initially, these words refer to their own mother or father or hands or feet. Infants begin to produce their first words when they are approximately one year old.[3][4] Infants' first words are normally used in reference to things that are of importance to them, such as objects, body parts, people, and relevant actions. Also, the first words that infants produce are mostly single-syllabic or repeated single syllables, such as "no" and "dada".[4] By 12 to 18 months of age, children's vocabularies often contain words such as "kitty", "bottle", "doll", "car" and "eye". Children's understanding of names for objects and people usually precedes their understanding of words that describe actions and relationships. "One" and "two" are the first number words that children learn between the ages of one and two.[5] Infants must be able to hear and play with sounds in their environment, and to break up various phonetic units to discover words and their related meanings.