Final answer:
Gripping a nail with pliers while hammering it into hard materials reduces its effective length and vibrations, thus preventing it from bending.
Step-by-step explanation:
The old carpenter's trick to prevent fingernails from bending when being hammered into tough materials involves gripping the nail firmly in the center with pliers. The reason this technique works is that it reduces the effective length of the nail that can act like a lever. When you grip the nail with pliers, it stabilizes the nail, making it more difficult to bend or veer off course due to the support provided by the pliers at the point where force is applied. Additionally, this support reduces vibrations in the free length of the nail, lowering the chance it will bend under the hammer's impact.