Bound by the Coulomb interaction forces alone, atomic nuclei and electrons combine to form an incredible variety of molecular systems. When molecules react, they undergo chemical transformations leading to rearrangement of atoms within molecules or transfer of atoms between molecules. This process may absorb or release energy; however, the energy change in a chemical reaction is much smaller than the total energy of the Coulomb interaction in a molecule (Fig. 1). This makes chemistry a game of small numbers and a chemical reaction a very complex process to study. Much of our current understanding of chemical reaction dynamics is due to the development of the technology for producing and colliding molecular beams [1]. A molecular beam is a gaseous ensemble of molecules moving as a whole in the laboratory frame. The molecules are often prepared with a narrow distribution of internal energies (up to a few kelvin) and a low density. When two molecular beams collide, molecules react under single-collision conditions and the reaction products scatter in a particular direction, where they can be detected by a variety of techniques. By varying the angle between the crossed beams, it is possible to tune the collision energy of the molecules. Molecular beam experiments, however, have two significant limitations: they only probe the outcome of a chemical reaction, providing no direct information about the actual process of bond breaking and bond making [2], and, because the density of molecules is usually undetermined, they cannot measure the absolute rate of a chemical reaction. The latter is crucial for calibrating theories of elementary chemical reactions. Two groups have now taken a completely new approach to study chemical transformations of molecules: Steven Knoop and colleagues at the University of Innsbruck in Austria and the Austrian Academy of Sciences, in collaboration with Jose D’Incao at JILA and Brett Esry at Kansas State University, both in the US, reporting in Physical Review Letters[3], and Silke Ospelkaus and coFIG. 1: Experiments with ultracold molecules probe chemical reactions in a completely new, previously unattainable, temperature regime, where the translational energy of molecules is much smaller than the energy of hyperfine interactions and the de Broglie wavelength of molecules is much larger than the typical size of molecules. The scale above shows the de Broglie wavelength of Cs2 molecules in units of Bohr radius. (Illustration: Carin Cain)