High-purity deuterium is an essential ingredient for next-generation nuclear fusion and other scientific applications, but getting D2 isn’t easy. It’s made either by electrolysis of heavy water followed by extraction or via distillation at a chilly –249 °C. Looking to sidestep these expensive and energy-intensive processes, scientists have sought to separate H2 and D2 with porous materials by a process known as kinetic quantum sieving. With this method, pores of a certain size preferentially confine the heavier isotope D2 over lighter H2 because of differences in their zero point energy—a quantum mechanical property. The problem is that the pores need to be very small, on the order of 2 Å wide, but materials with pores that small fill up quickly, so they don’t take up much D2. A team led by Andrew I. Cooper of the University of Liverpool and Michael Hirscher of the Max Planck Institute for Intelligent Systems used organic synthesis to address this problem (Science 2019, DOI: 10.1126/science.aax7427). By adjusting the apertures in structurally similar cage molecules, they were able to create cages with small pores for sieving D2 and cages with large pores for storing D2. They cocrystallized these materials, which resulted in a material that has excellent selectivity for D2 and high D2 uptake.