A 4-year-old boy in Watertown, N.Y., is growing an extra skeleton due to a rare genetic condition, the Watertown Daily Times reported Thursday. Shane Terry has fibrodysplasia ossificans progressiva, or FOP, which causes bone to form in muscles, tendons, ligaments and other connective tissue. In time, bridges of extra bone will develop across his joints, eventually restricting movement and forming a second skeleton. Eventually, his body will lock up.