A swimmer in a pool of syrup moves differently than in clear water: each stroke is harder, but you can push off more strongly. Roughly the same goes for single-celled organisms — tiny rowers that propel themselves through fluid using flagella. Scientists filmed Chlamydomonas algae and Salmonella bacteria swimming, and from their motion calculated the force they exert on the water at each moment.

It turned out that the thrust is not a simple wave but a superposition of several waves with different decay rates. This is like a musician playing a chord rather than a single note: the individual sounds blend, but each is heard in its own way. Previous models described only one note, so they poorly predicted where a cell would go. The new model, built on real data, accounted for the whole chord and showed that at intermediate fluid viscosity, cells spread fastest.

The secret is that the cell senses the density of its environment and changes the character of its strokes. In too-thin water, the stroke is wasted; in too-thick fluid, there's not enough force; somewhere in between, the fastest spreading occurs. In this respect, Chlamydomonas is more powerful than Salmonella: its 'motor' produces femtowatts — a thousand times more, though still vanishingly small for us.

This approach helps understand how bacteria spread in soil or how cancer cells carve a path through tissues. The viscosity of the environment turns out to be not just a backdrop but a tuning knob that the cell itself adjusts to its advantage.