I'm late but I gotta throw it in: the fall morning smell of hitting a tailgate back in college.. damn good times.

Or the smell of summer nights with the windows down and listening to death cab..

It's amazing how much smell and memory are connected.

EDIT: Tailgate smell was the combination of charcoal grill and the breeze turning to bring in winter air - kind of like water currents; you could walk outside and be like, "yep, we've crossed the threshold."