I am the only thing that always tells the truth. I show off everything that I see. I come in all shapes and sizes. So tell me, what must I be?

The object that always tells the truth, shows off everything it sees, and comes in all shapes and sizes is a mirror. This clever riddle points to the fundamental nature of mirrors: they reflect light precisely as it strikes their surface, offering an unbiased representation of whatever is placed before them. Whether it's a calm pool of water or a highly polished surface, a mirror's primary function is to return an image, making visible what is otherwise unseen from that perspective.

The concept of mirrors dates back thousands of years, with the earliest forms being naturally occurring obsidian, a volcanic glass polished to a reflective sheen, found around 8,000 BC in Anatolia. Ancient civilizations in Mesopotamia and Egypt later developed mirrors from polished metals like copper and bronze. The Romans further advanced mirror technology by using glass to cover reflective metal surfaces, improving their durability and clarity. Modern silvered-glass mirrors, similar to what we use today, were invented in 1835 by German chemist Justus von Liebig, who applied a thin layer of metallic silver to the back of a glass sheet.

Mirrors are not just for admiring one's reflection; their applications are incredibly diverse. Flat, or plane, mirrors are common in homes for daily grooming and decor, offering true-to-size reflections. Convex mirrors, which curve outward, provide a wider field of view, making them useful in vehicle side mirrors, security systems, and at blind street corners. Conversely, concave mirrors curve inward and can magnify images or focus light, finding uses in shaving mirrors, telescopes, and even solar concentrators. This variety in form and function truly showcases how mirrors come in all shapes and sizes, each designed to manipulate light in specific ways to serve different purposes.