October 20 is World Osteoporosis Day. Osteoporosis is the most common skeletal disease that affects people's health. What is osteoporosis? How to judge and prevent osteoporosis? How to supplement calcium scientifically? People's Daily Online invites Chen Hailing, Deputy Director and Chief Physician of Osteoporosis Department of Beijing Jishuitan Hospital affiliated to Capital Medical University, to answer your question online.