When a system exchanges energy with a constant-temperature environment, the entropy of the surroundings changes. A lattice model of a fluid thermal reservoir can provide a visualization of the microscopic changes that occur in the surroundings upon energy transfer from the system. This model can be used to clarify the consistency of phenomena such as crystallization or similar phase transitions with the second law of thermodynamics; in those phenomena, students intuitively grasp that the system entropy decreases, but may not have a clear picture of how it is compensated by an increase in the reservoir entropy. The model may be used in the classroom to visually demonstrate how processes in which the entropy of the system decreases can occur spontaneously; specifically, it shows how the reservoir temperature affects the magnitude of the entropy change that occurs upon energy transfer from the system.