Layers of the Earth
The earth has 3 basic layers--the crust, the mantle, and the core. Texts will emphasize the thinness of the crust. Making models is a wonderful way to demonstrate this, though I found my kids were particularly impressed by this demonstration.
The earth is not perfectly spherical, and the crust of the earth is irregular, so exact numbers are not really possible, but these are the ones we can use for this activity:
Radius = 6378 km
Distance to core edge = 2890 km
Distance to mantle edge = 35 km (from Wikipedia)
Take a measuring tape at least 6 m long; have someone hold it in place at the "center" of the "earth." First measure out to 2.73 m. Holding chalk and the edge of the tape together, draw a circle, or at least an arc; this first line is the edge of the core. Now bring the tape out to 6m and draw a second arc; this second line is the edge of the mantle. Ask your kids, "How thick is the crust in this model? How much longer do I extend the measuring tape to draw the crust?" Have them mark where they think the line would fall.
Answer: 3 cm
For advanced students, have them calculate the ratios. I choose to have the edge of the mantle to be 6 m, so how large is the full radius for the model? The actual distance from the center of the earth to the mantle's outer edge edge is 6378 km - 35 km = 6343 km. Now set up a ratio:
distance to outer mantle (6346) is to earth's radius (6378) as model distance to outer mantle (6 m) is to model radius (rm).
6346/6378 = 6/rm --> (6378)(6) = (6346)(rm) --> (6378)(6)/6346 = rm
rm = 6.03
You can set up a similar ratio to check the edge of the core.