If you ever feel like you are 【D6】_________ something again when you know it is actually the first time, you are having "déjà vu

You may be standing at the top of a mountain, about to ski for the first time in your life. Suddenly you have a quick flash of memory. Your mind tells you that you have been in this exact same spot before. Though you know it’ s impossible, all the sight, sounds, and smells seem familiar. This phenomenon is given the French name "déjà vu", meaning "already seen". While déjà vu causes anxiety in some people, others find it exciting and enjoyable. Although no one knows for certain what causes this feeling of familiarity, many people have suggested explanations for it.
    One suggestion is that when we experience déjà vu, we are actually recognizing elements of dreams from our past. Another possible explanation is that the feelings of familiarity are caused by something we have seen, read, or experienced before. Take the earlier example of the mountaintop: you may feel you have skied before because of a movie you saw or a book you read.
    Some scientists believe that déjà vu occurs when the brain mixes up past and present information. This creates an illusion of familiarity. They think this may be caused by a chemical in the brain not being at its correct level. Scientists have tried to determine the exact chemical involved but have so far been unsuccessful. Around 7 in 10 people report having experienced some form of déjà vu. Studies show that more incidents occur in people between 15 and 25 years old than in any other age group. So you don ’ t need to feel threatened or anxious by its occurrence because it commonly happens.