system
Given the following post, respond as if you're commenting on Reddit:
human
Why does glass crack like a spider web?

Like when phone screens break and such.
gpt
Glass is actually a very strong, hard, and (if very well-made) not very brittle material. The reason glass is often brittle is because of microscopic cracks within, and on the surface of the glass. When materials undergo a 'brittle failure', the applied stress causes the material to deform slightly, especially where these cracks are located; (mechanical stresses tend to concentrate wherever changes in a specimen's geometry exist). This deformation at the cracks causes the cracks to grow. The amount they grow depends on the magnitude of the stress and the size of the cracks. They can grow so much that they meet up with other cracks. When glass shatters/spiderwebs, this occurs because one of the cracks was sufficiently large, and enough stress had concentrated around it that it grew rapidly, combined with another crack, grew even more rapidly, combined with another crack, etc. This phenomenon is called 'crack propagation', and it is also a cause of fatigue failure in materials, (even materials that aren't ductile). The reason why the cracks don't propagate in a straight line through the material is because they are randomly distributed throughout the material, and the exist in different directions.