Birds, fish, and other creatures form amazing, undulating swarms of individuals. How these collectives comes together and move continues to fascinate scientists. Here, researchers look at simple particles with two “instructions,” if you will. One causes the particle to self-navigate toward a target; the other causes short-range repulsion if the particle gets too close to another one. With only these two simple guidelines, a flock of these particles forms complex, ever-changing flows! (Image and video credit: M. Casiulis and D. Levine)