Unlike white fat cells, which store lipids as a single large lipid droplet, brown adipocytes feature multiple smaller lipid droplets in their cytoplasm. This facilitates the high rates of lipid metabolism that are necessary for the heat-generating function of these cells. Barneda et al. studied how a brown fat-specific protein, CIDEA, mediates the fusion, and thus the growth of lipid droplets. They demonstrate that an amphipathic helix within the CIDEA carboxyl terminus is first inserted into the membranes of droplets, followed by the formation of intermolecular CIDEA complexes on adjacent droplets to facilitate their aggregation. Finally, lipids are transferred between droplets in a process that depends on the interaction of the CIDEA amphipathic helix with phosphatidic acid on lipid droplet membranes. These data shed light on the mechanisms of lipid droplet fusion and size regulation in brown fat. To better understand the physiological differences in lipid droplet growth between brown and white fat, it would be interesting to study how CIDEC — the equivalent of CIDEA in white adipocytes — mediates lipid droplet fusion.