Did you ever try to measure a smell?

You know the smell of warm, buttered popcorn. The pungent, somewhat sweet scent that precedes rain. But could you begin to describe these aromas in detail? Your nose has some 400 olfactory receptors that do the work of translating the world's estimated 40 billion odorous molecules into an even higher number of distinct scents your brain can understand. Yet although children are taught that grass is green and pigmented by chlorophyll, they rarely learn to describe the smell of a freshly cut lawn, let alone the ozone before a storm.

Alex Wiltschko opens a black plastic suitcase and pulls out about 60 glass vials. Each contains a different scent. One smells starchy with soft floral notes, like jasmine rice cooking. Another brings to mind ocean air and the white rind of a watermelon. One is like saffron with hints of leather and black tea.

Researchers have long known that the chemical structure of the molecules we inhale influences what we smell. But in most cases, no one can figure out exactly how. Scientists have deciphered a few specific rules that govern how the nose and brain perceive an airborne molecule based on its characteristics. It has become clear that we quickly recognize some sulfur-containing compounds as the scent of garlic, for example, and certain ammonia-derived amines as a fishy odor. It turns out that structurally unrelated molecules can have similar scents.

Alex Wiltschko began collecting perfumes as a teenager. His first bottle was Azzaro Pour Homme, a timeless cologne he spotted on the shelf at a T.J. Maxx department store. He recognized the name from Perfumes: The Guide, a book whose poetic descriptions of aroma had kick-started his obsession. Enchanted, he saved up his allowance to add to his collection. "I ended up going absolutely down the rabbit hole," he said.

Google has its own perfume--or at least one team of the company's researchers does. Crafted under the guidance of expert French perfumers, the mixture has notes of vanilla, jasmine, melon, and strawberries. "It wasn't half bad," says Alex Wiltschko, who keeps a vial of the perfume in his kitchen. Google's not marketing that scent anytime soon, but it is sticking its nose into yet another aspect of our lives: smell. On Thursday, researchers at Google Brain released a paper on the preprint site Arxiv showing how they trained a set of machine-learning algorithms to predict molecules' smell based on their structures.