“Déjà vu” describes the uncanny sensation that you’ve already experienced something, even when you know you never have.Say you go paddleboarding for the first time. You’ve never done anything like it, but you suddenly have a distinct memory of making the same arm motions, under the same blue sky, with the same waves lapping at your feet.Or perhaps you’re exploring a new city for the first time and all at once feel as if you’ve walked down that exact tree-lined footpath before.