People recognize that red and blue make purple (unless they're colorblind or just plain stubborn), but not everyone realizes that scents can combine to make other scents.  Here's an easy example:  yesterday, I was making cinnamon french toast casserole and, at the same time, rehydrating raisins on the stove.  I paused, because it smelled exactly like Sun Maid Cinnamon Raisin Bread.  

But it's more complex than that.  Occasionally, I've stepped into the walk-in fridge and smelled something completely different, something for which we don't even have the ingredients.  The combination of other dishes cooling creates the perception of a third, unrelated scent.

There's science behind this.  The chemical diacetyl is present in butter, and shows up in wine that has undergone malolactic fermentation.  It's responsible for that buttery taste in Chardonnay, but there isn't actual butter involved, just the same volatile chemical interacting with retronasal sensors.  (Most of what we consider "taste" is actually smell that occurs within the mouth.  The tastebuds can only perceive tactile sensations and the basic tastes:  sweet, salty, bitter, sour and umami.)

I've recently become aware that I have a much keener sense of smell than I'd ever thought.  This is especially weird because growing up (and still), I had miserable allergies.  I was used to being stuffed up and not relying on my nose.  In fact, I still breathe solely through my mouth.  (Yep ... I'm a mouth-breather.)  So I'm used to thinking that I had a subpar sense of smell.  Maybe it's that I concentrate on it more than most people; maybe it's that I appreciate it more.  It might even be connected to my writing:  I've always tried to include smell, taste and touch in my descriptions, so I'm used to pinpointing and labeling.

However it comes about, I do notice the interaction of smells.  I haven't yet picked out a pattern as to what combinations create what results ... after all, the human nose can pick out 10,000 scents (at least), which makes millions of potential blends.  An olfactory rainbow waiting to be discovered.