Optional: start by reviewing the states of matter, while asking students to find examples in the classroom. Solids have a fixed shape and are hard. Liquids can change shape, but always take up the same amount of space (same volume) as they flow in their container. Gases change shape and fills the space they are in (so change volume). Some students might mention plasma, the 4th state of matter.
The particles (molecules or atoms) are arranged differently in different states of matter. Tell students that they will model the particles in each of the three states of matter. They are each a molecule. Ask them to stand in an open space of the classroom.
In a solid the molecules are packed tight, held together by strong bonds. Ask students to model a solid by linking arms with their neighbours, so that they are packed tight together in a group. The individuals can jiggle a little, but the group maintains its shape.
As a solid gains energy the molecules gain energy and can move around more. They are still bonded to each other, but more loosely. Ask students to model a liquid by spreading apart and moving around more, but always touching at least one other student with an outstretched arm. The bonds between molecules break and form continuously, so that the group stays together but can move and change shape, like a liquid.
As a liquid gains even more energy the molecules gain enough energy to move completely apart and evaporate. Ask students to break all bonds with each other and move around the room, spreading out to fill the room. Students can be asked to lose energy and become a liquid again (condensation), then a solid (freezing).
For younger students, simplify the language:
Stand up close together, arms linked. Stay still. You are not moving. You are stuck together, so you cannot change shape. I can see what shape you are. Which state are you? Solid.
You have a bit more energy. Now you can move around a little more. Walk around, but always stay touching at least one other person. So you can change shape but stay the same size. Which state are you? Liquid.
Now you have even more energy. You can now move around even more, let go of each other. You now can change shape and size. Which state are you? Gas.
Videos of students acting out states of matter:
Video graphic of states of matter: https://www.youtube.com/watch?v=s-KvoVzukHo
Image of particles in the three states: http://wps.prenhall.com/wps/media/objects/943/966267/fig10.gif
Longer cartoon video of the particles in solids, liquids, gases at https://www.youtube.com/watch?v=guoU_cuR8EE