Mold

Mold explained in an easy to understand way:

Imagine losing your keys in a snowstorm and only finding their shape outlined in the snow after they melt away. This is similar to how a rock fossil mold is formed when an organism buried in sediment dissolves, leaving behind an impression. Just like the keys leave an imprint in the snow, the dissolved organism leaves a mold in the rock, capturing its shape even though the original object is gone.