A Cartesian conception of material space views each part as external to every other part: "partes extra partes." Because two material things cannot occupy the same space, each of it exists in itself, separated by a boundary from everything else, including other things. This ontology is the origin of thinking the world in terms of self-identical entities, separated from everything else surrounding them. This leads to the borders or boundaries that separate each thing from what it is not. The areas near them are borderlands. Those capable of living on either side are "boundary workers" and "boundary crossers" or "wizards." And "immutable mobiles" are things that without change are crossing boundaries and affording connections distinct parts of the social world. Although the concepts of borders and boundaries have had considerable currency in the scholarly literature, there have been alternate conceptions of the social and cultural world that have instead focused on the impossibility to have anything like pure parts (e.g., culture, language) and the pervasiveness of mixtures, hybridity, "métissage," and heterogeneity. Any interest in coming to grips with the aliveness of life leads us to a fluid ontology, where the figure of thought is something alive, in movement, an event. The purpose of this article is to articulate a perspective on boundaries and borders through the lens of a fluid ontology of the social. A concrete case study from a physics classroom is used to exemplify the fact that life has the characteristic of a fluid, where there always are mixture and hybridity, and where the impending future always includes, unpredictable, novelty. The very boundaries and borders that are stable things and concepts in other theories are but the result of abstractions from a multitude of flows and counter-flows. The fluid perspective puts into question the usefulness of a range of theoretical conceptions--including "actor network," "third space," and "cultural appropriation"--all of which are based on an ontology of self-identical things.