There are different ways fossils can form. Fossils can be molds, or imprints left from something buried. The actual structure decomposes, but the shape is left behind. A shell imprint in a rock is an example. Fossils can also be casts. Casts are molds that fill in over time. They show a three-dimensional shape of the structure that used to be there. A walnut mold that fills in with minerals is an example. Plants and animals can also be preserved when they are trapped in certain circumstances. Tar, ice and amber have all trapped animals for thousands of years. Amber comes from tree resin, or sap, which hardens. Insects from long ago have been found preserved in it. In California, tar pits preserved many animals, including a mastodon and a saber-toothed cat. What is the difference between a mold and a cast?

A. Molds are body fossils, and casts are trace fossils.
B. A mold is an imprint, and a cast is a filled-in imprint.
C. Molds are made of amber, and casts are made of tar.
D. Molds come from plants, and casts come from animals.