Task: Sentence Ordering
Input (English): Steps: (1) The gravity of the sun pulls its mass inward (2) The pressure forces atoms of hydrogen to fuse together in nuclear reactions (3) The energy from the reactions gives off different kinds of light (4) The light travels to the Earth.  
Where does the following step fit into the given list of steps: There is a lot of pressure on the Sun 
Output (English):  After step 1.