Strength of Glass

Glass is a strong material. Like most materials, glass can be bent until a certain limit.
Imagine bending a long rod of glass. If you release the tension before this limit, the rod returns to its original shape: the deformation is elastic.
If you pass the limit, the glass breaks.

Why does glass shatter?
The strength of glass is only slightly affected by composition but is highly dependent on the surface condition. If stress is applied on the damaged surface, the stress at the damaged points will be increased and the glass will shatter.

Glass does not age quickly: glass windows remain clear and undamaged after many years of exposure to the elements.