The author's grandpa and grandma take daily walks. Courtesy of Kris Ann Valdez My grandpa recently celebrated his 90th birthday. He attributes his long, happy life to many things, including a full social calendar and generosity. He and my grandma also recently started working out and doing bodyweight exercises daily. According to my grandpa, his cardiologist calls him his "poster child." He had a quadruple-bypass open heart surgery 25 years ago — a surgery expected to increase his life only by