Hugh Pickens writes: "Researchers are starting to discover the simple rules that allow swarms of thousands of relatively simple animals to form a collective brain able to make decisions and move like a single organism. To get a sense of swarms, Dr. Iain Couzin, a mathematical biologist at the Collective Animal Behaviour Laboratory at Princeton University, builds computer models of virtual swarms with thousands of individual agents that he can program to follow a few simple rules. Among the findings are that swarm behavior has patterns common to many different species, that just as liquid water can suddenly begin to boil, swarm behavior can also change abruptly in character, and that just a few leaders can guide a swarm effectively by creating a bias in the swarm's movement that steers it in a particular direction. The rules of the swarm may also apply to the cells inside our bodies and researchers are working with cancer biologists to discover the rules by which cancer cells work together to build tumors or migrate through tissues. Even brain cells may follow the same rules for collective behavior seen in locusts or fish. "How does your brain take this information and come to a collective decision about what you're seeing?" Dr. Couzin says. The answer, he suspects, may lie in our inner swarm."