Flux balance analysis is how systems biologists predict what a metabolic network will actually do: not by simulating every enzyme's kinetics, but by asking which steady-state flux distribution maximizes a cellular objective, like growth. This simulator renders a small branched network — uptake feeding a biomass reaction and a byproduct reaction — as both an animated flow diagram and, more importantly, as the convex polytope of feasible steady-state fluxes that the network's mass-balance equation and capacity bounds define in three dimensions. Adjust the three reaction bounds to reshape the polytope, adjust the objective weight to tilt the linear objective between biomass and byproduct, and watch the optimal solution jump from vertex to vertex exactly as it does inside a real linear-programming solver — because in FBA, as in any linear program over a convex polytope, the optimum always sits at a vertex.