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.

Comments:

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.