Task: Sentence Ordering
Input (English): Steps: (1) When water freezes it becomes 10% bigger, or takes up 10% more space (2) The force of the pressure exerted on the rock surrounding it is enough to wedge the walls of the crack farther apart, thus expanding and deepening the crack (3) Clay expends when wet like ice does (4) This causes rock to break apart (5) Some plants like mosses and lichens are capable of growing without soil on bare rock (6) When they do this their roots penetrate pores and crevices and cause rocks to split apart as the roots force their way down through the rocks  
Where does the following step fit into the given list of steps: As water expands it puts great pressure on the walls of anything containing it, including any rocks which happen to be surrounding it 
Output (English):  After step 1.