This article describes an activity that can be used as an inquiry-based laboratory or demonstration for either high school or undergraduate chemistry students to provide a basis for understanding both vapor pressure and the concept of dynamic phase equilibrium. The activity includes a simple setup to create a closed system of only water liquid and vapor where volume can be varied. As volume is varied, the vapor pressure remains constant. However, this challenges common misconceptions and can be contrasted with the same system containing only gaseous air. Through discussion, students are challenged to understand the molecular basis for the difference between the two systems and conclude that the liquid-vapor system maintains constant pressure due to phase equilibrium. Through further reasoning, the idea of dynamic equilibrium is illustrated and students come to see that particles are in constant movement within a system even though it appears stationary from the macroscopic view. (Contains 1 table and 2 figures.)