A teenager has worsening knee pain and a sunburst X-ray at the distal femur. This is osteosarcoma until proven otherwise — peak in teens, metaphysis of long bones (distal femur, proximal tibia), sunburst periosteal reaction and Codman's triangle on X-ray. Biopsy confirms; treatment is neoadjuvant chemotherapy + limb-sparing surgery. Night pain and a growing mass in a teen is never 'growing pains' — the X-ray pattern is the alarm, and delay costs limbs and lives.