butterfly effect SPOILERS u have been warned!
 

SPOILER u have been warned a 2nd time
SPOILER u have been warned a 3rd time
if he goes back and changes everything how come it is still in his journals to read again and go back to change over again

and when he gave the metal shard to Lenny what it leads up to is my favorite part (for people who have not seen the movie and ignore the 3 SPOILER warnings i wont tell what it leads up to)