Stirring gold nanoparticles in water with liquid mercury leads to alloy particles, in which the amount of mercury simply depends on the reaction time. On page 3259, Stijn F. L. Mertens and co-workers show that the large difference in cohesive energy between the alloying elements causes the slow inward diffusion of mercury over a fixed distance, independent of the mercury content. After a few days, the particles consist of a pure gold core (indicated in red in the image) surrounded by a solid solution of the two elements.