The idea that time flows is firmly entrenched in the manifest conception of time. Such is the strength of this feeling that many distinguished thinkers assume that positing a basic physical or metaphysical flow is the only action fully respecting it. I'll argue that this is a mistake. The cosmologist Gold emphasized that we need to explain the flow, asserting that "there can be a self-consistent set of rules that would give a beast this kind of phoney picture of time." While I don't think flow need be illusory -- instead it could be like color -- I want to take up this interdisciplinary explanatory project. Appealing to the hard facts of life in a relativistic world, our environments, and our psychology, I develop a theory of why "beasts" like us assert that time flows.