What are fingernails made of and why do they grow?

The answer

Fingernails are composed primarily of alpha-keratin, a tough structural protein also found in hair, hooves, and rhinoceros horns, arranged in tightly packed, flattened layers. Nails grow from the nail matrix, a tissue beneath the cuticle where rapidly dividing cells are pushed outward, harden through keratinization, and die to form the visible nail plate. Fingernails grow at an average rate of 3.5 millimeters per month, roughly twice as fast as toenails. Nails on the dominant hand grow slightly faster due to increased blood flow from greater use. Nail growth rate is affected by age (slows with aging), season (faster in summer), nutrition, and health. The pale half-moon shape at the base of the nail (lunula) is visible where the nail matrix cells have not yet fully keratinized and flattened.