A sphere rolls down a chute. What shape should the chute be to make the descent as quick as possible? Three centuries ago, the answer was the cycloid — a curve resembling the arc of an upside-down bridge. But submerge the chute in a thick liquid, and everything changes: the fluid slows the sphere, and the fastest path becomes something else.

Researchers ran an experiment: a sphere rolled down an inclined plane inside a tank of syrup. Cameras tracked its trajectory, while calculations sought the best route for two goals — arriving first or expending the least energy. In air, both goals led to the same cycloid. In syrup, the paths diverged.

The fastest descent is almost a straight line, but with smooth curves at the start and end. The sphere seems to accelerate along a flat road, then gently brakes before the finish. The most energy-efficient path, however, is an S-shaped curve with an inflection in the middle: the sphere first veers to one side, then comes back, like a skier who zigzags to avoid picking up too much speed.

This isn't just a playful exercise with spheres. Inside living cells, organelles move through dense cytoplasm; volcanic lava flows down slopes; and drugs are carried through the bloodstream. Everywhere the medium is viscous, the fastest and most energy-efficient paths are not what intuition from a frictionless world would suggest.