This simulation helps learners visualize how molecules behave in solids, liquids, and gases. Add or remove heat and watch the motion of the molecules as they change phase. Push the pump and change the volume of matter in the closed container and watch the pressure gauge respond. More advanced students can compare the potential energy graphs for neon, argon, oxygen, and water — which all have different interaction potential. Editor’s Note: This particular activity would be well paired with the PhET “”Gas Properties”” simulation, which goes into more depth to explore the behavior of gas molecules in a closed container. Together, the simulations promote understanding of gas laws, states of matter, phase change, and kinetic theory. See Related Materials for a link to an exemplary lesson plan developed to accompany “”States of Matter”” simulation. This item is part of a larger collection of simulations developed by the Physics Education Technology project (PhET).