It’s happened to us all. Maybe you’re walking down the street, or sitting on the bus, or even just sending an email – then suddenly, you realise you’ve been here before. Déjà vu can be a puzzling thing. It literally means already seen in English, and it remains pretty mysterious as to why we feel it. Now, scientists have a couple of theories. Sanam Hafeez, a clinical psychologist, told Fox News: "It refers to the eerie and distinct feeling that one has already experienced the current situation or event, even though it’s a new and unfamiliar occurrence. "It feels like a powerful wave of familiarity with the present moment as if the person is re-living a past experience. "Some suggest it may be linked to how memories are processed in the brain, potentially involving delays or errors in memory retrievals." She added that it may be because the brain is processing information through multiple pathways at the same time, creating the illusion of a memory when you are living in the present moment. She said: "Regardless of the precise mechanism, déjà vu is a transient and common experience that lasts only briefly, affecting people of all ages and not considered a pathological condition. "While it remains a puzzle, déjà vu continues to be a fascinating facet of human consciousness." "It is also important to note that déjà vu is not associated with any particular medical or psychological condition. It is usually a brief and transient experience and is considered a normal aspect of human perception and memory." About two-thirds of people in good health experience déjà vu during their lifetime, according to WebMD, though it is more likely to happen to people aged 15 to 25. The website explains: "A familiar sight or sound can trigger the feeling. You may walk into a room in a building you’ve never visited yet feel like you know it intimately." Health.com adds: "People with more education, those who travel a lot and people who can recall their dreams are also more likely to experience déjà vu." How to join the indy100's free WhatsApp channel Sign up to our free indy100 weekly newsletter Have your say in our news democracy. Click the upvote icon at the top of the page to help raise this article through the indy100 rankings.