on core members of a certain category. For example: 'cat' may only refer to the family cat and no other cat, or 'dog' may refer to certain kinds of dogs that the child is exposed to. Verb meaning: when a pre-school child hears the verb 'fill', he understands it as the action 'pour' rather than the result, which is 'make full'. Dimensional terms: the first dimensional adjectives acquired are big and small because they belong to the size category. The size category is the most general one. Later children acquire the single dimension adjectives, such as, tall-short, long-short, high-low. Eventually they acquire the adjectives that describe the secondary dimension, such as thick-thin, wide-narrow and deep-shallow. From birth to one year, comprehension (the language we understand) develops before production (the language we use). There is about a 5-month lag in between the two. Babies have an innate preference to listen to their mother's voice. Babies can recognize familiar words and use preverbal gestures. Within the first 12–18 months semantic roles are expressed in one word speech including agent, object, location, possession, nonexistence and denial. Words are understood outside of routine games but the child still needs contextual support for lexical comprehension. 18–24 months Prevalent relations are expressed such as agent-action, agent-object, action-location. Also, there is a vocabulary spurt between 18 and 24 months, which includes fast mapping. Fast mapping is the babies' ability to learn a lot of new things quickly. The majority of the babies' new vocabulary consists of object words (nouns) and action words (verbs). 30–36 months The child is able to use and understand why question and basic spatial terms such as in, on or under. 36–42 months There is an understanding of basic color words and kinship terms. Also, the child has an understanding of the semantic relationship between adjacent and conjoined sentences, including casual and contrastive. 42–48 months When and how questions are comprehended as well as basic shape words such as circle, square and triangle. 48–60 months Knowledge of letter names and sounds emerges, as well as numbers. By 3–5 years, children usually have difficulty using words correctly. Children experience many problems such as underextensions, taking a general word and applying it specifically (for example, 'cartoons' specifically for 'Mickey Mouse') and overextensions, taking a specific word and applying it too generally (example, 'ant' for any insect). However, children coin words to fill in for words