Inside a living cell, droplets float that don't mix with the surrounding fluid, like oil in water. These droplets aren't just passive clumps; they actively press on membranes and make them bend. Scientists observed this process in living plant cells, reproduced it in a test tube, and simulated it on a computer.

It turns out that the membrane's shape depends on the tension at the droplet's surface. If the tension is high, the membrane flattens into a sheet. If the tension is lower, the membrane rolls into a tube or a cup. The transition between these shapes isn't always reversible: to go back from a cup to a tube, you need to overcome an energy barrier, and the membrane 'remembers' its previous shape.

This mechanism explains how 'onions' form inside vacuoles in plant cells — large fluid-filled bubbles. The vacuole membrane invaginates under the influence of droplets and forms cup-shaped structures. By controlling the surface properties of the droplets, the cell can mold membranes into desired shapes at the right time.