All the neurons within this microscopic roundworm are highlighted, with the large cluster at one end representing the brain. Even worms have free will. If offered a delicious smell, for example, a roundworm will usually stop its wandering to investigate the source, but sometimes it won't. Just as with humans, the same stimulus does not always provoke the same response, even from the same individual. New research at Rockefeller University, published online today (March 12) in Cell, offers a new neurological explanation for this variability, derived by studying a simple three-cell network within the roundworm brain.