Solids, strangely, are sometime difficult to describe without comparing them to some other state of matter. Essentially solids hold their shape regardless of what sort of container they are in. Their molecules are tightly packed so they don't really move around a whole lot.
Talking about solids with a preschooler is really kind of boring - they get it. Totally and intuitively understand that the block, ball, or lego just isn't going to flow into the container no matter what container you put it into.
That's were oobleck comes in. And this is where the mess comes in.
Oobleck is a substance that isn't really a liquid and isn't always a solid. It is called a Non-Newtonian Substance. And if you want to really get your geek on, check this out. It is a good explanation of how this sometimes fluid works and does so in an accessible way.
But, wait, there's more!
This is the COOLEST THING EVER - and I don't say that lightly. I am so jaded when it comes to cool science tricks and this one totally rocks. The fact that I love the blog and kids do the experiment doesn't hurt. What they did was put oobleck over a subwoofer and watched it dance.
Why is this cool?
This was a great demo of the fluid characteristics of oobleck.
You can visualize the sound waves.
And they mixed color.
Coolest thing ever.