whatA cloud of bacteria climbs toward food it cannot see and cannot steer toward. Drag the food and the cloud follows, streaming after it like a comet.

whyThis is how E. coli finds sugar: a random walk with one bit of memory. The piece shows that a search can succeed with no sense of direction at all, only a sense of whether things just got better.

howA bacterium swims in a straight run, then tumbles into a completely random new direction. It remembers one thing: is the attractant stronger now than a moment ago? If yes, it tumbles less often. Runs that happen to point uphill last longer than runs that point downhill, and the cloud drifts up the gradient. Concentration is drawn as dashed isolines; the trace at top right is the cloud's mean distance to the food.