Seeds of change: An unusually fast cation-exchange reaction at the nanoscale is an efficient tool for precise control of the nucleation of nanocrystals. As a model system, 3.2–14-nm PbSe nanocrystals are produced by the nucleation reaction of Pb2+ with transient SnSe nuclei (see picture). The synthesis allows surface derivatization of the PbSe nanocrystals with various ligands.