Is there a way to revive an old potato? I was making mashed potatoes the other day and one of the potatoes I had was a little old. It was not as firm, and was slightly wrinkled than the potatoes I'd just bought. I didn't use it at all.

Is there a way to revive an old potato? For instance, with celery, I sometimes stick it in a cup of water for re-crisping.
Cut out any green parts then remove the skin. If you can see any remaining green parts after the skin is removed then cut them out too. What you'll have at this point will likely look and feel like any other peeled potato except with bits cut out. If it still feels soft or looks unusual throw it out, otherwise its fine. Just a normal potato whose skin had gone a bit wrinkly and soft.