The first observation of arrested empty liquids — sparse networks of bonded nanoparticles — is reported online in Nature Materials this week. The finding will make it possible to synthesize lightweight materials that are exceptionally stable and do not age. Colloidal suspensions — liquids containing dispersed nanoparticles — typically undergo extremely slow separation into two phases. Such an arrested separation is an unstable gel. But, over a period of seven years, Ruzicka and colleagues observed a colloidal clay that forms a low-density (that is, almost empty) liquid that does not phase separate and becomes a stable gel. They also found that the gel consists of a sparse network of nanometre-sized discs making T-shaped contacts with each other — that is, the rim of one disc touches the face of another — as predicted from computer simulations of colloidal particles that form a limited number of bonds.