If you take two paper strips, staple them into an X, then place them on a pencil top, you can make a hollow sphere.
If you spin that sphere, you can see the sides bulge out, just as the Earth bulges at it's center from the force of it's rotation.
And while we're spinning things...
I have the kids find out which eggs are raw and which are hard boiled by spinning them.
If you stop the egg briefly, a raw egg will start spinning again because of the momentum of the liquid yolk. If it's hard boiled, it stays still.
I had them identify a hard boiled one, then, trusting to their skills, I chopped it in half with a large knife!
Fortunately, they were right! The egg is a good model for the Earth: the shell is proportionately as thick as the crust, the white is the mantle, and the yolk is the core. To be perfectly correct, the yolk would be a little smaller (and have an inner core).
Crunching the shell a bit, you can model how the tectonic plates connect to each other.
I floated them on water, then put the pan on the stove. To visualize the convection currents, I dropped in a little dye.
I had to use a push pin to keep the sponges from sticking to each other, but they separated and wandered about the pan, pushed by the heat driven motion of the water.
The kids had a general idea that the continents on our planet had moved in the past (they knew about Pangea), but the idea that they were still moving (at the rate your fingernails grow) and that the world would look different in a few million years were new and fascinating.
There is a great interactive animation here. And here is a straightforward video:
We looked a bit at plate boundaries using books to show the plate interactions.
Transform: the plates are pushing past each other (causing earthquakes).
Divergent: pulling apart like the Great Rift Valley or the plates in the center of the Atlantic.
Convergent: pushing together to form folded mountains like the Himalayas and the subduction zones in the Ring of Fire.
I showed the mountain making by pushing together layers of towels - this was a good introduction to the layering of rocks and sediment as ecosystems change.
In my case, the blue towel was a layer of sea sediment, the yellow was desert sand, and the brown was a forest.
Very often convergent plates lead to one plate going over and one going under (subduction). The plate underneath melts back into Magma, but often presses up against the other plate causing volcanoes!
We used small soda bottles surrounded by salt dough.
The kids built them on the plates, and I loaded them up with a few tablespoons of baking soda.
Then the kids named them and we took them outside.
I'm using vinegar spiked with soap and red paint.