This first experiment is one of the simplest but also one of the coolest things you can make with stuff you probably already have in your kitchen cupboard. If you don't have the ingredients, a quick trip to the grocery store with a dollar and you'll be all set!
Here is what you need:
Put the cornstarch into the bowl, slowly add the water. When you get the mixture to the right consistency it should flow like a liquid but act like a solid when you squeeze it or put pressure on it. Isn't it awesome?! Oobleck is a non-newtonian fluid which means it is a liquid and a solid at the same time. When you squeeze the oobleck it acts like a solid because you are pushing the molecules together. When you let it go it flows like a liquid because the molecules are free to flow and tumble over each other as liquids do.
There are 3 types of matter solid, liquid and gas. In solids the molecules are lined up in a definite structure and vibrate slowly back and forth in place. In a liquid the molecules are not lined up in a definite pattern and are free to flow and tumble over each other. They move more quickly then in a solid. In gases molecules are moving very quickly and can escape their containers.
For a good demonstration of all 3 states of matter and the physical changes that they make when they change state, put some ice cubes in a pot on the stove. You can see the definite structure of the ice cube and that the molecules inside, while vibrating slowly are lined up. As the cubes start to melt into liquid you can see that the molecules behavior change. Liquids take the shape of their container and if you tip the pan you can see that the molecules flow freely over each other. (By the way the reason the behavior is changing is because you are adding energy to the molecules as you heat them and that is why they are speeding up) As you continue to add energy to the molecules they will move faster and faster until they get so fast they turn into steam, which is a gas, and fly out of the pan.
If you want to demonstrate this point even further have children act out being a molecule in each state of matter. You or another child will be the source of heat, perhaps you are a hair dryer. Have the children start out as a solid, they should stand shoulder to shoulder and move back and forth ever so slightly. Add energy to them in the form of heat and they turn into a liquid, now they are moving around each other changing places etc. Finally as you add more heat they get so much energy that they escape from their container and run around the room. In real life the molecules would keep moving until they ran out of energy, which might not be a good idea for your kids, or maybe it would be :)
Oobleck is also a good tool to show how lava forms. There are 3 basic layers of the Earth, the crust the mantle and the core. The mantle is a malleable layer that is sort of like a soft solid. The mantle remains solid until the pressure from the crust is removed, such as when tectonic plates move. When the pressure from the crust is removed the mantle turns into a liquid form and since the liquid form is less dense then the rocks above it it rises up through the rock above it and forms a volcano on the surface of the Earth.