To introduce the three states of matter, give each student or pair of students a small plastic baggie containing an ice cube. Next, take the students and their baggies outside, preferably on a sunny day. Ask them to watch what happens to their ice cube. When the ice cube has melted, place an Alka-Seltzer tablet in the water and make sure the baggie is sealed tightly. Tell the students to watch what occurs. Throughout the changes to the water, you can explain that the water is changing from a solid to a liquid and to a gas. Since they have been documenting what happened to the ice cube and to the melted water, you can use their collected data to start talking about the properties of each state.
If you want to show students a physical example of liquid changing into a gas, all you need is an uninflated balloon and a two liter bottle of carbonated soda. Place the bottle on a desk or other location so all of the students can watch. Have the balloon ready then open the soda bottle. Quickly place the end of the balloon over the opening of the bottle. Try to cover as much of the bottle's neck as possible with the balloon so no gas escapes. Then the class can watch as the released carbon dioxide from the bottle inflates the balloon. Ask the students for their explanations as to what is happening.
The reason for differences in the three states of matter is their molecular properties. For older science students, you can give them a visualization of this difference. Get an empty petri dish and an overhead projector. Fill the dish with BBs so that the entire bottom is covered and none of the tiny silver balls can move. Show this on the overhead and explain that it represents the molecules in solid matter. Then take out about half of the BBs so they can move around on the bottom of the dish and explain that this represents liquid matter. Finally, remove more of the BBs and show they can move a lot in the dish just as the molecules can in gas matter.
For younger science students, physically acting out the processes of evaporation and condensation can help them understand the basics of the states of matter, as well as the water cycle. Go outside to the slide on the playground. Have all of the students stand at the base of the slide and explain that each of them is a droplet of water in a puddle. Now you, an assistant, or another student acts like the sun and causes some of the water to evaporate. Those students you select must climb the steps of the slide and stay at the top to form a cloud. Finally, those students can slide down as rain and can end up back in their puddle. Then you can tell the water droplets in the puddle to huddle together as closely as they can because now they are freezing into ice.