You can demonstrate how liquid matter can be transformed into solid matter by changing sugar water into crystals. Fill a glass with 2 cups of warm water and add 2 cups of granulated sugar. Stir the solution well. Tie a button onto a piece of twine that is as long as the glass. Attach the other end of the twine to a pencil or a pen. Set the pencil or pen across the top of the glass, allowing the twine to sink into the solution until the button reaches the bottom. After around one week, the liquid will have changed into solid crystals.
Changing a liquid into a gas can be achieved by heating water until it boils. Place a pot or a saucepan on a stove and add 2 cups of water. Heat the water over medium-high heat until it starts to boil. The water can be seen as water vapor rising out of the pot. Be careful not to allow students to touch the rising gas, or steam, as it can be extremely hot and can burn skin.
Changing a solid state of matter into a gaseous state is more complicated, and should only be accomplished by an adult wearing protective gloves. Purchase a piece of dry ice from a grocery store or supermarket. Use tongs to pick up the dry ice. Place the dry ice into a bowl, then add a small amount of water onto the ice. The solid ice will instantly change into gas, releasing so much gas that it creates a cloud of vapor. Do not allow students to touch the dry ice, as it can cause frostbite and ice burns on skin.