What is déjà vu?

You ask: What deja vu?

Strange feeling hit Caveman: this moment happen before! But it never did. Science tribe still argue why. Leading idea: brain keep two piles — one for 'happening now', one for 'feels familiar'. Usually they match up. In deja vu, the familiar pile light up by mistake, no real old memory behind it. Brain shout 'Caveman know this!' but cannot find where from. Part of brain near ear (temporal lobe) may misfire, like spark jump to wrong stone. Other tribe say eye send scene to brain twice with tiny gap, so second copy feel like old memory. Happen more when Caveman tired, and near two of every three people feel it sometime in life. Cave wisdom: deja vu not Caveman seeing future — just a familiar-feeling fire with no memory to hold behind it.