A stock and flow diagram is a quantitative model of the systems thinking approach and is explained prior to a causal loop diagram in this book. It is a basis of system dynamics and is compared to a bathtub with hot water. By adding equations and values to the model, it presents simulation results. As examples of stock and flow diagrams, a population model is introduced. Changes in variables by hypothetical scenarios are also shown to enable a fundamental understanding of the relationships between the variables.