One of the most impressive aspect of the regeneration process is the completeness and perfection of the replacement limb. Replacement limbs are perfect in every way, remaining indistinguishable from ordinary limbs. Some other vertebrates can replace part of a missing structure, or at least begin the process but it is almost always imperfect. Lizards for example can often replace their tails, but the replacement tails are missing vertebrae. Another feature of the axolotl is that it can regenerate missing structures for its entire life. Humans for example have been documented to replace the very distal tips of their fingers. This ability is documented to work best in children, but persists in a less perfect form throughout life. Finger tip amputations that are sutured shut with mature skin are inhibited from regenerating. Most animals show some regenerative ability early in life, but lose that ability as they age. Part of the challenge for human regeneration research then is to learn how to unlock the regenerative potential within us.