Sedimentary rocks are formed when mineral particles are squeezed together and the spaces between the particles get filled in. Soft minerals like limestone actually dissolve in water, so the water carries minerals into the tiny spaces and fills them up. Once all the spaces between the particles are filled in you have a new rock but if you cut it open you can still see the individual rocks that stuck together to form the new rock. Bones and shells are made of minerals so they can be part of that process just like a rock. They become part of the new rock but you can still tell that they used to be bones.
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