fossils – the preserved remains of a dead prehistoric plant or animal embedded in rock

To remember what a fossil is, use the following mnemonic:

The fossils were preserved. You could see the dead prehistoric plants and animals in the rock.

Fossils are the preserved remains, traces, or impressions or organisms that lived millions of years ago, providing us with direct evidence of ancient life on Earth. 

When plants, animals or other living things died in the distant past, their remains were sometimes quickly buried by sediment such as mud, sand and volcanic ash, which protected them from decay. Over millions of years, more layers of sediment builds up on top, creating enormous pressure and heat that gradually turns the sediment into sedimentary rock. During this process, the organic materials from the surrounding rock, essentially turning the remains to stone while preserving the original shape and structure. 

Some fossils form when organisms become trapped in substance like amber (fossilised tree resin) or tar, which can be preserve even soft tissue and tiny details. By studying the rock layers where fossils are found and using techniques like radiometric dating, researchers can determine the age of fossils and piece together the story of how life has developed and changed over billions of years, giving us crucial evidence for understanding evolution and Earth's history.