I'm tall when I'm young and short when I'm old. What am I?

The riddle perfectly describes a candle because a brand new candle, fresh out of its packaging, stands tall and full of potential. This represents its "youth." As it is lit and begins to fulfill its purpose, the flame consumes the wax and wick, gradually diminishing its height. Over time, as more of its material is burned away, the candle becomes shorter and shorter, embodying its "old age" until it is completely gone or too small to burn safely.

Candles have a rich history, dating back thousands of years. Early evidence of candle use can be found in ancient civilizations, with some attributing the earliest use to the Ancient Egyptians around 3000 BC, who made rushlights by soaking reeds in animal fat. However, the ancient Romans are often credited with developing wicked candles by repeatedly dipping rolled papyrus in melted tallow or beeswax. Throughout history, candles have been crafted from various materials, including beeswax, animal fats (tallow), and later, spermaceti from whales, before the widespread adoption of paraffin wax in the 19th century.

The science behind a burning candle is a fascinating interplay of heat, fuel, and oxygen. When the wick is lit, the heat melts the wax nearby. This liquid wax is then drawn up the wick through capillary action, similar to how a plant draws water. The heat of the flame vaporizes this liquid wax, turning it into a hot gas that combines with oxygen in the air to create a self-sustaining flame. This combustion process releases light, heat, water vapor, and carbon dioxide, continuously melting more wax to fuel the flame until the candle is fully consumed.