When I visited the Tin Building, a sprawling, upscale food hall in New York's Seaport District, restaurant critic was on my mind. Wells was largely complimentary, but, he said, something was missing. While the European markets that served as inspiration are packed with smells, the squeaky-clean Tin Building had no scent.

As I walked through the complex, I kept sniffing the air. Wells was right. From the fishmonger to the flower stall, the place was odorless.

The Tin Building could learn from the fast food business. Smell is key to the way we experience food, and the world-class marketing teams behind the biggest fast food brands know it. Using scent to reel in customers is nothing new: food stalls have wafted scents toward customers since the beginning of history.

It doesn't take a marketing genius to know that coffee scent plus customer cravings equals Starbucks sales. It's as simple as two plus two equals four. But what new is the science of scent — and the way companies use it to influence customers' moods and memories.

What is scent marketing? Scent marketing isn't limited to restaurants. Hotels, clothing stores, and amusement parks use it, too. Entire marketing agencies are devoted to helping brands pick a signature scent.

The oldest, ScentAir, got their start perfuming Disney parks — a famous example of scent marketing. According to ScentAir, customers spend up to 23% more money in scented stores. In restaurants, a subtle scent can disguise unpleasant orders and pro.