I heard about phantom limbs a while ago. When I was in middle school I was on a sort of army/navy SEAL kick and I read a lot about guys who lost limbs in combat or other circumstances. A phantom limb is when someone can still feel the lost limb. Sometimes their toe itches (even though they lost their foot), or it feels like his arm fell asleep (even though they lost their arm years before). Pretty wild stuff.
We talked about phantom limbs in physics today. We're on the section about optics and my professor talked about how a therapist used optics to help an amputee overcome his phantom limb syndrome. This guy had lost his left hand but it still hurt him months after everything had healed. He told the therapist that it felt like his left hand was clenched in a fist with the fingernails digging viciously into his palms. The pain he felt was constant. He was having trouble sleeping at night and concentrating during the day. His life was consumed with the pain he felt in his left hand ... which he didn't even have. It's important to note that the pain he felt was real. The nerves coming from his wrist were sending signals to the brain and the brain interpreted the signals as pain from his missing hand. The signals received by the brain were no different than any other pain signal that our nerves send.
The therapist tried mental exercises, coaching the guy to try to imagine opening his hand. It was too hard. No matter what he did, the man couldn't imagine his hand opening and relaxing. The therapist thought for a while and came up with the following solution.
He had the guy sit like this with the mirror in between his good arm and his phantom arm. Now when he looked at his missing hand, it looked as if a real hand was there. The therapist had the man open and close his hand over and over again. Over time, the pain went away and the man could move on with his life.
Pretty amazing, no? If you want to read more about it, just Google "phantom limb mirror therapy" and it will pull up some scholarly papers written on the subject.