eli5 Why do human fingernails grow so long?
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Fingernails are made of Keratin which is very strong, strong enough not to wear or crack under most our exertions. So it just keeps growing until it eventually cracks and breaks off, but that means it can keep going a very long length if a person is very gentle with their fingernails. But most opt to simply use a trimmer to manage the length.
Which is the same with almost all keratin protrusions. Most animals will have instinctive behavior to wear or crack their nails(claws) to keep them from getting too loud. Typically involving scratching at hard objects like trees or hard-packed dirt. Cats may enjoy scratching a pole, but that enjoyment is from a evolved behavior that keeps their claws at a near optimal length.
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