Century eggs are made by preserving the egg (typically duck eggs, but sometimes quail or chicken) in a mixture high in sodium and hydroxide to create a chemical reaction to alter the pH of the egg. Traditionally, the eggs are soaked in a tea mixture, coated in a paste made of wood ash, salt, and calcium oxide, and wrapped in rice hulls or husks to cure. They’re then left in an airtight container for weeks, but sometimes months, to fully ferment. This process causes the egg whites to turn black (or dark brown) and the egg yolk to dark green. While some people say it looks like the egg has gone bad, it’s perfectly safe (and delicious) to eat!