Lipid droplets are key organelles regulating cellular energy metabolism. They are implicated in various metabolic disorders such as obesity, diabetes or neurodegenerative diseases. Understanding the formation of these organelles is important in biology and metabolism. Lipid droplet formation starts in the phospholipid bilayer membrane of the endoplasmic reticulum and is mainly triggered when cells feel excess nutrients around them. In these conditions, neutral lipids, i.e. fat molecules, are synthesized and accumulate within the bilayer membrane leaflets where, over a critical concentration, they possibly demix to form a lens in the bilayer. This lens grows by acquiring more lipids to bud off and form a spherical droplet. The aim of my talk is to present how basic experimental physics of soft matter enabled to start understanding the formation mechanism of cellular lipid droplets.