I started out talking about the structure of the Earth (the crust, mantle, core, etc.) and we talked about how the Earth is not a perfect sphere.
To look at why the Earth bulges in the middle, we assembled these nifty contraptions.
You cut out two long thin strips of construction paper and punch
You thread the holes onto a pencil or pen so that you form a ball with the pen just inserted a little way into the ball (we stapled the part where the strips cross to give some stability).
When you twirl the pen, the ball flattens because of centripetal force.
Since we were already spinning things, I brought out a tray of 6 eggs - half hard boiled and half raw! I asked the kids to figure out which were which...
The trick is to spin the egg and stop it briefly. If the egg is hard boiled, it will stay stopped. If it's raw, it will start turning again because the inertial motion of the liquid inside will still be spinning.
Now for the act of faith! Once they agreed that a particular egg was hard boiled, I chopped it in half with one really quick hard blow from a heavy kitchen knife. They were correct!
If you do it fast enough, you get a really clean cut and you can use the eggs layers as a model of the Earth with the shell being the crust, the white the mantle and the yolk the core. The core is not to scale (and you don't have an inner core), but the shell is. What I wanted the kids to see was that the crust is a very small, very thin layer of the Earth.
I then crunched up the egg a bit to show how the crust is in pieces (tectonic plates).
We looked at how the plates move using sponge continents in a pan of water. We put the continents together, then heated the pan on the stove. I added a bit of dye to visualize the heat driven currents.
This worked really well! The only trick is to use push pins so the sponges don't get stuck together by surface tension.
A great interactive animation of this happening over the history of the Earth (including the future) is here. And here is also a short video showing the last 600 million years.
We talked about how the plates move against each other, demonstrating with notebooks.
Transform: moving side ways, causing earthquakes.
Divergent: spreading apart to form a chasm (like the Rift Valley) or the chasm may open into magma which can form mountains (like the Atlantic mid-ocean ridge).
Convergent: smashing together to form folded mountains like the Himalayas (which we showed with layers of colored towels), one plate usually goes under the other(subducts), melting back into magma, and sometimes causing volcano inducing pressure (like most of the Ring of Fire).
Did someone say volcanoes?!
We built ours out of salt dough around a very small (1 cup) soda bottle. I preloaded each volcano with a few tablespoons of baking soda, and then we took them outside.
Each kid gave their volcano a name, then I poured in the vinegar (spiked with soap and red paint).