Science Question--about fossils?

What the other person said is 95% correct, however he did make a light mistake. When any organic material is imprinted in stone, the organic material is decomposed by microscopic organisms which exist inside the rock, thus, the organic material is no longer present. How imprints are formed, is organic matter is encased in sedimentary rock. the pressure on the organic material against the rock leaves an imprint on the rock's surface. while this proses does take millions of years, it is actually very easy to comprehend.