Task: Sentence Ordering
Input (English): Steps: (1) Dead organic material accumulates on the bottom of oceans, riverbeds or swamps, mixing with mud and sand (2) Over time, more sediment piles on top (3) The resulting heat and pressure transforms the organic layer into a dark and waxy substance known as kerogen (4) The molecules break up into shorter and lighter molecules composed almost solely of carbon and hydrogen atoms (5) It will turn into either petroleum or natural gas  
Where does the following step fit into the given list of steps: Left alone, the kerogen molecules eventually crack 
Output (English):  After step 3.