“Tip of the tongue (or TOT) is the phenomenon of failing to retrieve a word from memory, combined with partial recall and the feeling that retrieval is imminent. The phenomenon's name comes from the saying, “It's on the tip of my tongue.” The tip of the tongue phenomenon reveals that lexical access occurs in stages.
People experiencing the tip-of-the-tongue phenomenon can often recall one or more features of the target word, such as the first letter, its syllabic stress, and words similar in sound and/or meaning. Individuals report a feeling of being seized by the state, feeling something like mild anguish while searching for the word, and a sense of relief when the word is found.”
Since those who experience it feel sure that they know the answer, it would seem to be a problem with accessing already-stored memory. The implication being that the fact that a memory has been 'stored' is itself also stored as a memory (perhaps analagous to an 'index card' in a library).
Despite extensive research, the mechanisms of the phenomenon are unknown - though there are several theories, inclcuding :
• Direct-access view • Blocking hypothesis • Incomplete-activation hypothesis • Transmission-deficit model • Cue-familiarity theory