The vastness and sparseness of space makes it a challenging environment to building complex molecules. In the interstellar medium, most complex molecules are believed to form in molecular clouds, pictured, on dust grains. Now, writing in Nature Astronomy, Masashi Tsuge and co-workers from Hokkaido University and the University of Tokyo have determined a new reactivity pathway for making such molecules, where carbon atoms can diffuse across an icy dust grain surface to meet and react with another atom, building molecular complexity.