I was working on my father's feet one time when he decided to have me look at his shoes. I picked up his hiking boots and looked inside. I said, "How long have you worn these boots?" His reply was that he had worn them daily for 13 years.
My next question startled him. I asked if he noticed the brown stain in the bottom of the shoe. He said, "Yes I did notice that. What is it?" I replied, "Blood". He said "Blood? How did that get there?" I reached inside and found the nail that had worked it's way up through the footbed. It was sharp.
He hadn't realized that every time he took a step in these boots he was being jabbed by this nail. He was dumbfounded. "Why didn't I notice it?" he asked.
That part of the question was easy. One, the nail had worked it's way up gradually. And then the other factor is that the feet are very good at pain killing. This can backfire as the pain killing qualities sometimes mask what should be attended to before the blood starts to flow.
My mother could not believe what happened next. He eliminated 12 pairs of shoes that were will beyond their expiration date.