What it shows:
A simplified model crystal with non-rigid inter-atomic bonds. You can show that solids really do vibrate, distort and expand.
How it works:
A cubic lattice of 3×3×3 15cm diameter Styrofoam™ spheres linked by 3cm steel springs. The springs are epoxied to corks embedded in the Styrofoam.
This is much more fun than those little rigid-bond models. We've calculated that one mole of this crystal would have twice the volume of the Earth.