Potatoes are a living organism. And as such, the tubers (potatoes) have an inert life support system which, similar to cut flowers, will draw from its water supply (potatoes are approximately 79% water) to survive. Whether staying in the ground for an extended time or stored too long in a pantry, potatoes will eventually begin their reproductive cycle (sprout, skin greening) and dehydrate as they draw in stored moisture. Fortunately, if the potatoes are still relatively firm and not overly-dehydrated they can absolutely still be used. Just peel away any green skins and discard any sprouting that may have occurred and prepare as desired.