The mental lexicon is not fixed like a dictionary, is not composed of an alphabetical list of words but it is dynamic in its nature. It is a continuously developing and widening interwoven structure. We cannot study or analyze it directly but we can find clues how it works. These clues can support the evolvement of strategies, techniques and activities to acquire L1 or later L2 vocabulary items. The knowledge about the ‘human word-store’s’ nature, structure and organizing rules is utilizable to decide which strategies are appropriate at different ages. Teachers are also responsible for getting learners on to the metacognitive approach of vocabulary learning, which makes the process of learning much easier. It is obvious that the metacognitive technique can be applied only at an elder age when the learner is mature and conscious enough to learn about the process of learning.