Here's why:

* Animals: The remains of animals, like bones, shells, or teeth, can get buried in sediment (like sand, mud, or volcanic ash). Over millions of years, the sediment hardens into rock, and the animal remains are preserved as fossils.

* Plants: Similarly, plant parts like leaves, stems, or roots can be fossilized. Sometimes, even entire trees can be preserved as fossils.

So, fossils aren't just from animals. They can be from any organism that lived in the past!