a, In particles larger than about 10 nanometres, plasmons emerge as collective oscillations of a gas of conduction electrons, and have a frequency that is uncertain (double-headed arrow) because of collisions among the electrons and between the electrons and the particles' atomic lattice. b, In particles smaller than 10 nanometres, plasmons are associated with quantum electron transitions between occupied and unoccupied energy levels. As a result, the plasmon frequency and its uncertainty, which Scholl et al.1 accurately measured, are larger than those for bigger particles.