<P> Cracks are linear openings that form in materials to relieve stress . When an elastic material stretches or shrinks uniformly, it eventually reaches its breaking strength and then fails suddenly in all directions, creating cracks with 120 degree joints, so three cracks meet at a node . Conversely, when an inelastic material fails, straight cracks form to relieve the stress . Further stress in the same direction would then simply open the existing cracks; stress at right angles can create new cracks, at 90 degrees to the old ones . Thus the pattern of cracks indicates whether the material is elastic or not . In a tough fibrous material like oak tree bark, cracks form to relieve stress as usual, but they do not grow long as their growth is interrupted by bundles of strong elastic fibres . Since each species of tree has its own structure at the levels of cell and of molecules, each has its own pattern of splitting in its bark . </P> <Ul> <Li> <P> Old pottery surface, white glaze with mainly 90 ° cracks </P> </Li> <Li> <P> Drying inelastic mud in the Rann of Kutch with mainly 90 ° cracks </P> </Li> <Li> <P> Veined gabbro with 90 ° cracks, near Sgurr na Stri, Skye </P> </Li> <Li> <P> Drying elastic mud in Sicily with mainly 120 ° cracks </P> </Li> <Li> <P> Cooled basalt at Giant's Causeway . Vertical mainly 120 ° cracks giving hexagonal columns </P> </Li> <Li> <P> Palm trunk with branching vertical cracks (and horizontal leaf scars) </P> </Li> </Ul> <Li> <P> Old pottery surface, white glaze with mainly 90 ° cracks </P> </Li> <P> Old pottery surface, white glaze with mainly 90 ° cracks </P>

Name two places in nature where you would find hexagon