Many molecules exist in two mirror-image forms—like left and right hands—that look identical but can behave very differently, especially in biological systems and pharmaceuticals. Distinguishing between these enantiomers (also called chiral molecules) is a longstanding challenge in science and technology. A useful analogy is that of a screw and a nut: A right-handed screw fits only into a right-handed thread, while a left-handed one will not engage properly.