Here's a more detailed explanation:
* Nails and Hair Composition: Both nails and hair are primarily composed of a protein called keratin. The keratinocytes (cells that produce keratin) in the nail matrix (where nails grow from) and hair follicles are alive, but as they produce keratin, they get pushed further away from the blood supply. Eventually, they die and become densely packed with keratin, forming the hardened, visible nail and hair that extends beyond your skin.
* Lack of Nerves: Dead cells do not have nerve endings. Nerve endings are the sensory receptors that transmit pain signals to your brain. Since your nails and the visible parts of your hair consist of dead cells, there are no nerve endings to trigger pain when they are cut.
* Pain When Cutting Too Short: If you cut your nails or hair too short, you might feel pain. This is because you've cut into the living tissue where the nails and hair are still being produced. For nails, this would be the nail bed underneath the visible nail. For hair, this would be down inside the hair follicle in the scalp. These areas *do* have nerve endings, hence the pain.