The same set of parts can assemble into different shapes, just as magnetic beads can form a small cube or a large sphere. That's how hepatitis B virus proteins behave: they build two capsids of different sizes. One is made of 90 protein pairs, the other of 120. Both are stable and appear both in living cells and in the test tube.

The difference lies in salt. When the solution has many ions, proteins more often choose the small capsid. Few ions, and the large one assembles. Scientists built a model where salt modulates the strength of attraction between proteins, like glue that becomes stronger or weaker. This determines which shape is more favorable.

The model accurately predicted how many large and small capsids would form at different salt concentrations. The researchers also tracked how the ratio of shapes changes over time when proteins are scarce. This is akin to how, in a crowd, circles of different sizes gradually form — depending on how readily people join hands.