Odd To Know on Instagram: "A signal mirror is one of the most effective non-electronic survival tools because it uses specular reflection to concentrate sunlight into a high-intensity beam that can be seen for over 20 miles. Unlike a standard household mirror, a survival signal mirror typically features a retroreflective aiming hole in the center; this creates a “fireball” or “aiming spot” on your hand or a nearby surface, allowing you to align the reflected sunlight precisely with a distant tar