Here's how it works:

1. Rapid Burial: An organism dies and is quickly buried in sediment (like mud, sand, or volcanic ash). This prevents decomposition and scavengers from destroying the remains.

2. Mineralization: Groundwater rich in dissolved minerals (like silica, calcite, or pyrite) seeps through the porous sediment and replaces the organic matter in the bones, teeth, or shells with minerals. This process happens gradually, molecule by molecule, preserving the original shape and structure.

3. Formation of Fossil: Over time, the sediment hardens into rock, and the mineralized remains become a fossil.

While other methods exist, permineralization is the most common due to the widespread availability of groundwater and the ease with which minerals can replace organic material.