Q: In this task, you are given a paragraph, and your task is to construct a question from the given passage. Construct question in such a way that (i) it is well explained in the paragraph, (ii) the words may/may not be present in the paragraph, (iii) it should start with what/how/describe. Avoid creating a question that (i) is just relevant to one or two sentences, and (ii) not relevant to passage. 
A plant of animal dies in a watery environment. Is buried in mud and silt. Soft tissues quickly decompose leaving behind hard bones or shells. Over time sediment builds over the top and hardens into rock. As the bone decays mineral seeps in replacing it. Fossils are formed. 
A:
How are fossils formed?