Donnie has the ability to see into the future.. Frank - the giant rabbit - gets him out of bed on the night that the Airplane engine is supposed to fall on him and kill him. However, by seeing what happens when he DOESN'T get killed (meets gretchen, falls in love, etc etc etc .. learns about time travel, meets grandma death) Donnie sees what a little blip in the course of time can do - basically gretchen's death - and chooses to sacrifice his own life for the lives of the ones he loves.
I too, love the "What are feces?" part. And nothing beats Cherita !! Chut up!!