Abstract: In creatures ranging from birds to fish to wildebeest, we observe the collective and coherent motion of large numbers of organisms, known as ‘flocking’. In this talk, I'll use the hydrodynamic theory of flocking to explain why a crowd of people can all walk, but not point, in the same direction. Along the way I'll illustrate how one goes about formulating a hydrodynamic theory for heretofore unconsidered states and system, using powerful techniques from theoretical condensed matter physics such as hydrodynamic theories, the gradient expansion, and the renormalization group, and using concepts from fluid mechanics.