皮蛋 is a Chinese preserved egg, often called 'century egg' or 'thousand-year-old egg' in English. It's made by preserving duck, chicken, or quail eggs in a mixture of clay, ash, salt, and lime for several weeks to months. The yolk turns creamy and dark green, and the white becomes a dark brown jelly with a distinctive ammonia-like flavor. It's commonly served sliced as a cold appetizer with soy sauce and sesame oil, or used in congee (皮蛋瘦肉粥).