How do fossils form?

Fossils form when organic remains are quickly buried and mineralized. After an organism dies, rapid burial by sediment (sand, mud, ash) is crucial to protect the remains from decay and scavengers. Over long times, water percolating through sediment dissolves original organic materials and deposits minerals in their place (permineralization), turning tissues into stone. Hard parts (bones, shells) fossilize most readily. Thus fossilization requires two key factors: rapid burial and durable material. Given these, the buried remains endure geological timescales, becoming preserved as fossils geologists study.