You hear an old song, and instantly you're transported back in time; the memories which surround the song are accessed in a flash.
You see a movie which you once saw years ago - and suddenly you're back in that mindset, feeling the same emotions as you felt the first time round.
My latest journey back was provoked by a young boy playing - badly - on a recorder. I was immediately back in primary school, where my biggest decision was whether to join the Recorder club or the school choir. In Sixth class the recorder group accompanied the choir at school performances so I had the best of both worlds.
I can still play most of Simon and Garfunkel's music on the recorder, a heap of Christmas carols, and the theme from Exodus. Not that I DO, mind, but I could, if I wanted to....