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One approach is to point out that a solid has a definite size and shape.  If you put a solid in a container, the solid doesn't change its shape to match the container, and it doesn't change its size. So, for instance, if you put a football in a big punch bowl, the football won't change its shape to match the bowl.  And it doesn't change its size.

A liquid, in contrast, will take the shape of whatever container it's in.  However, the liquid will not change its size (volume).

A gas takes both the shape and size of the container it's in.  That's hard to visualize since most gases are invisible, but you could add some smoke to the air in a container, such as a jar with a lid (e.g., by lighting and then blowing out a candle to make a little smoke).  They can then see that the air carries the smoke particles to all parts of the container.  Thus the gas -- air in this case -- fills the container.

Extension:  Some liquids are very slow-moving (a property that scientists call viscosity).  Silly Putty is an example.  If you make a little tower or other tall shape with Silly Putty, at first it will look like a solid, but if you look at it again in 5 minutes or 15 minutes, you will see that it has started to deform.  And if you come back in an hour or two, you'll see that it's in a puddle, or, if it's in a container such as the plastic egg it comes in, it will take the shape of that container after a while.  This shows that Silly Putty is actually a very slow-moving, or viscous, liquid.

I hope that helps.  

Matt

Hey, I think this can be a pretty difficult topic to cover with a younger age group but I think it is possible if you try to connect the topic to their everyday life. I would use something like water to explain the states of matter. You can create ice and show how the shape of ice (as long as it remains cool) keeps its shape. Then you can have water and show that liquid changes its shape by pouring it into different sized containers (food coloring could be added so that water is more easily visible). Then with has you could show them hot water and have them feel the steam, but I would be careful with it so that students do not hurt themselves. I also think it is important that you use appropriate vocabulary with the age group introducing vocab should be paired with the demonstartion so that student can have a direct reference. Hope this helps good luck on the lesson! Autumn

The football is actually a solid, the air inside is a gas. I think you do need to bring that up. As science teachers, we sometimes become stumped when we look at objects because we know the inside is filled with a gas. Just like balloons. We think that if you put gas in a balloon that it will float. However, that depends on what kind of gas is filling the balloon. Since this is kindergarten, keep it straightforward. You are introducing them to content and it may be the first time that they have explored this topic. I made plastic cages with ping pong balls inside to differentiate solids, liquids, and gases. However, with smaller children that may be too much information for them. Gases and liquids can be poured, solids can not. Have you looked at the resources here on NSTA?

Here is an example of a lesson: http://www.brighthubeducation.com/pre-k-and-k-lesson-plans/61387-properties-of-matter-kindergarten-lesson-and-activity/ I work with 3-5th grade and even at that age they have misconceptions about matter. Allow your students time to figure out what is happening when they pour a liquid from say, a tall cylinder into a measuring cup. Have them pour it back and forth; is it still the same amount of water? Did it change its shape?