The horror of scratching a blackboard with fingernails, and aversion to the horrible sound of grinding a pebble must come from the fact that fingernails and teeth have no feeling. Digging in the ground for roots, aversion to the sensation of scratching stone would protect one's fingers. When biting a peach the squeak of the tooth on stone is a warning to prevent damage. If the sound meant nothing we might damage our teeth. Were it not for the aversion to scratching a stone we might damage our fingers. Even thinking about these things "sets one's teeth on edge".
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Laboratoire Kronk Nailcare
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