The makings of permeable membranes: A challenge in making membranes is finding ways to control the pore structure during the fabrication process or through postfabrication treatment. The deposition of a zeolitic imidazolate framework material called ZIF-8 onto an alumina support gives a dense, impermeable material. However, when Ma et al. exposed this material to vapors of 2-methylimidazole, it transformed into a porous material able to separate propylene from propane. Science , this issue p. 1008