the age of eighteen months. These might include words such as, milk, water, juice and apple (noun-like words). Afterwards they acquire 12 to 16 words a day. By the age of six, they master about 13 to 14 thousand words. The most frequent words include adjective-like expressions for displeasure and rejection such as 'no'. They also include social interaction words, such as "please" and "bye". There are three stages for learning the meaning of new words: Whole object assumption: A new word refers to a whole object. For example, when an eighteen-months old child sees a sheep and his mother points at it and says the word 'sheep', the child infers that the word 'sheep' describes the whole animal and not parts of it (such as color, shape, etc.). Type assumption: A new word refers to a type of thing, not just to a particular thing. For example, when the child hears the word 'sheep' he infers that it is used for the animal type and not only for that particular sheep that he saw. Basic level assumption: A new word refers to objects that are alike in basic ways (appearance, behavior, etc.). In other words, when the child hears the word "sheep" he overgeneralizes it to other animals that look like sheep by the external appearance, such as white, wooly and four-legged animal. Contextual clues are a major factor in the child's vocabulary development. The child uses contextual clues to draw inferences about the category and meaning of new words. By doing so, the child distinguishes between names and ordinary nouns. For example, when an object is presented to the child with the determiner "a" (a cat, a dog, a bottle) he perceives it as an ordinary noun. However, when the child hears a noun without the determiner, he perceives it as a name, for instance "this is Mary". Children usually make correct meaning associations with the words that the adults say. However, sometimes they make semantic errors. There are a few types of semantic errors: Overextension: When a child says or hears a word, they might associate what they see or hear as more generalized concept than the real meaning of the word. For example, if they say "cat", they might overextend it to other animals with same features. Underextension: It involves the use of lexical items in an overly restrictive fashion. In other words, the child focuses