We often say, "It's on the tip of my tongue," when we are sure we know a word but cannot recall it at that moment.

This is not merely a clever idiom; it reflects a real neurological phenomenon called "lethologica."

The term comes from the Greek words lethe, meaning "forgetfulness," and logos, meaning "word," though researchers usually refer to it as a tip-of-the-tongue state, or TOT state.

TOT states occur worldwide. A survey in 1999 found that over three-quarters of languages use a tongue metaphor similar to English to describe this experience. But why does it happen?

Producing language is a complex process in which the brain transforms abstract ideas into words.

Normally this works smoothly, explains Karin Humphreys from Canada's McMaster University.

"In a TOT state, however, the system breaks down and you become stuck partway through," she said in 2015.

Interestingly, TOT states can repeat for the same word.

A 2008 study showed that the longer someone struggles to recall a word before being reminded, the more likely they are to experience another TOT state when trying to remember it later.

Conversely, recalling the word on one's own reduces the chance of repetition, as found in a 2015 study. People are able to retrieve the intended word more than 90% of the time.

TOT states are more likely when individuals are tired and when they attempt to remember proper names.

Age also plays a role: a 1999 diary study found that TOT states occur about once or twice a week for young adults aged 18 to 24, but they are twice as frequent for those aged 80 to 92.

This phenomenon fascinates researchers because it shows how memory, language, and cognition can momentarily misalign, leaving us knowing that a word exists even when it temporarily slips from reach.