A century egg is made by preserving a quail, duck or chicken egg in a saline solution that consists of salt and clay, but may also include ash and rice hulls. Contrary to its name, a century egg only takes, at the most, a few months to make.
Century eggs are sometimes called thousand-year eggs or millennium eggs. The preservation process turns the egg yolks into a cheese-like substance while the white of the eggs turn into a dark-colored jelly.
These smelly eggs are known as "horse urine eggs" in Thailand because of the mistaken belief that the eggs are preserved with horse urine, and "pine-patterned eggs" in China due to the pattern that develops on the eggs during their preservation. They are a delicacy in many Asian countries and are sometimes eaten alone but more often served with congee or rice porridge. This delicacy has been around for hundreds of years and was an accidental discovery.
When century eggs are ready to eat, they develop a strong, ammonia and sulfur smell due to the fact that the egg's pH and sodium content are altered. They turn a dark gray color and are often served as a regular meal or at special occasions.