In this simple demonstration, students make predictions and then observe the transitions between liquid and gas states of matter.
Using water is a simple way to show the three states of matter, and also the easiest to show the transitions between each state.
At room temperature, water is in its liquid state. It can easily be converted to gas state by heating it up as its boiling point is only 100 degrees Celsius. It can also be frozen readily because its freezing point is below or at 0 degrees Celsius to illustrate the solid phase.