The original version of this story appeared in Quanta Magazine. After her adventures in Wonderland, the fictional Alice stepped through the mirror above her fireplace in Lewis Carroll’s 1871 novel Through the Looking-Glass to discover how the reflected realm differed from her own. She found that the books were all written in reverse, and the people were “living backwards,” navigating a world where effects preceded their causes. When objects appear different in the mirror, scientists call them chiral. Hands, for instance, are chiral. Imagine Alice trying to shake hands with her reflection. A right hand in mirror-world becomes a left hand, and there’s no way to align the two perfectly for a handshake because the fingers bend the wrong way. (In fact, the word “chirality” originates from the Greek word for “hand.”) Alice’s experience reflects something deep about our own universe: Everything is not the same through the looking glass. The behavior of many familiar objects, from molecules to elementary particles, depends on which mirror-image version we interact with. Mirror Milk At the beginning of Through the Looking-Glass, Alice holds her cat Kitty up to the mirror and threatens to push her through to the other side. “I wonder if they’d give you milk in there? Perhaps Looking-glass milk isn’t good to drink,” she says. Alice was onto something. Just over two decades before the book’s publication, Louis Pasteur discovered while experimenting with some expired wine that certain molecules can be chiral. They can come in distinct left-handed and right-handed structural forms that are impossible to superimpose. Pasteur found that, while they contain all the same components, the mirror versions of chiral molecules can serve distinct chemical functions. The pioneering French chemist and microbiologist Louis Pasteur discovered the chirality of biomolecules in the late 1840s. Photograph: Smithsonian Institution Librarie Lactose, the sugar found in milk, is chiral. While either version can be synthesized, the sugars produced and consumed by living organisms are always the right-handed ones. In fact, life as we know it uses only right-handed sugars—hence why the genetic staircase of DNA always twists to the right. The root of this “homochirality” remains one of the biggest mysteries clouding the origins of life.