Why does hearing an old song or smelling a familiar scent instantly transport you back in time?

Imagine walking into your childhood home and catching a whiff of a favorite meal cooking. In that instant, a flood of memories and emotions might wash over you this is nostalgia. It s often a bittersweet feeling: happy recollections tinged with a longing for the past. Centuries ago, nostalgia was literally considered an illness the term comes from Greek nostos (return home) and algos (pain), coined by a 17th-century physician to describe the ache of Swiss soldiers longing for home.