A piece of rope.

Some years ago, while riding along a street I'd been on many thousands of times, a strange smell floated past. A familiar smell. One that took me back 50yrs to my childhood when I used to root around grandpa's garage area. A short length of rope. Nasty, probably in that gutter for years. But it smelled identical to grandpa's garage. That old, awful, somehow comforting mix of oil, dirt, sweat, general crud, weathering, and whatever any number of animals chose to do with the thing. The old memories flashed through my head as I passed it. Took me minutes to find the thing, coming back to search for it. Weird, how certain smells can unleash a cascade of old memories like that.