Fossils

A fossil is evidence, in rock, of something that was once alive. These are pictures found online of different types of fossils.

These are cast and mold fossils because the one on the left is a hollow imprint of the swirly thing,  which means whatever living thing it was decayed and only this is left (making a cast);  the one on the right is a mold since it looks the same as the other one  only now the spaces are filled in with minerals.

This one is a trace fossil because it's not the actual remains or imprints of anything,  but it's a footprint preserved in the rock so it still shows proof of something alive.

This is an original remains fossil because the ant thing is completely preserved in amber,  which doesn't let it decay or anything, so it's still in its original state.