Nails start in the nail bed, a flat surface that is under your nails and extends about 10mm beyond where you can see them. When cells at the root of the nail bed grow together to form keratin, a nail is formed. Layers of keratin bind together and the nail slowly grow out from the root of the nail bed towards the end of your finger. Nails grow very slowly. The nails you see now won't be fully replaced by entirely new nails until 6 months or more from now.