Here's why:

* Shear force acts parallel to the surface of an object, causing it to slide or deform in a direction perpendicular to the applied force.

* This sliding motion can create internal stresses within the material.

* If the shear force is strong enough, it can overcome the material's strength, leading to fracture or failure.

Examples of shear force:

* Cutting a piece of paper with scissors: The blades of the scissors apply a shear force to the paper, causing it to tear.

* A nail being hammered into wood: The hammer's force drives the nail into the wood, creating shear stresses along the nail's axis.

* A bridge collapsing under heavy traffic: The weight of the traffic creates shear forces on the bridge's structure, potentially causing it to fail.

Other forces that can cause materials to break:

* Tensile force: Pulling or stretching a material apart.

* Compressive force: Pushing or squeezing a material together.

* Torsional force: Twisting or rotating a material around its axis.