Salt crystals react quite differently to mechanical stress. Rather than yielding to stress, they first resist, then either cleave cleanly along layers or crack and shatter.

This is because positive and negative charges alternate in the crystal lattice of a salt. In an undeformed salt crystal these charges interlock in a stable way.

But if one sheet of atoms is pushed over another, similar charges on the two sheets are brought closer together (see right).

The strong electrostatic repulsion between similar charges pushes the sheets apart, and the crystal shatters.

If the layers of a salt crystal are pushed past one another, the resulting repulsion by like charges will push the layers apart, thereby breaking the crystal.