Pain is when you go to sleep with a wet head and wake up with that dreaded splitting headache.
Pain is when you're tested on C4 after discussing a previous paper and then you left a question totally blank.
Pain is when you go to sleep on Friday and wake up 6am on Saturday for extra classes.
Pain is when you did all these and went home early, forgetting some event in college where you don't really bother attending but you still felt missed out though.
Pain is a feature of life. Life is an example of pain.