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.