The states of matter can best be described by the “Kinetic theory on matter” which basically says that all matter is made of molecules which are in constant motion and the amount of motion determines the state which will exist.
Solids have a definite shape and take up a definite amount of space, liquids take up a definite amount of space but have no definite shape, and gases have no definite space or shape.
The changing of one state to another involves the adding or removing of heat which affects the molecular motion.
A good explanation of the molecular motions in the various states is found on page 324 in “Physical Science” by Ramsey, Gabriel, McGuirk, Phillips, and Watenpaugh published by Holt, Rinehart, and Winston 1986.
It uses people in a theater to show the difference between the states. It starts with people sitting in their seats watching a performance. (solid). When the performance is over they begin to leave their seats. (liquid) Finally they leave the theater and move out in all directions. (gas)
This situation could also be used to explain the molecular actions with the addition or removal of heat.
A good activity showing the effects of heat would be heating iced water until it reaches it boiling point. Plotting a graph with time vs temperature is a beneficial activity which can show the boiling point, freezing point. and melting point of water.