Great question. Fossil is one of those classic words which means different things to different people. When most people think of fossils, they think of dinosaur bones or other, very old remains of ancient creatures. But scientists simply define fossils as the naturally preserved remains or traces of an animal or plant. So, you could argue that leaves on the ground or shells on a beach are fossils, and youd be right. In that sense, it takes almost no time to form a fossil.

However, leaves on the ground and shells on a beach probably arent going to last for very long theyll eventually decay or break into smaller and smaller pieces. For fossils to really last a long time, a couple of things need to happen. First, the plant or animal remains must be buried before they can be destroyed. A flood or volcano eruption, for example, might kill and bury a large number of animals and plants. Second, as sediments accumulate and pile up on top of the fossil remains, the fossils are often remineralized, in which the original material is gradually dissolved and replaced by other, stronger material. When you look at dinosaur bones in the museum, you usually are not looking at the original bone material, but at a remineralized replacement.

If you want to learn more about fossils, I strongly recommend that you go to the La Brea Tar Pits, in Los Angeles. They contain an amazing number of fossil mammals and other animals, all preserved because the animals were trapped by the Tar Pits and sank down into the ground. You can also go to the beach and start your own collection of fossil shells!

Good luck!