Title: Grow Up
Prue wishes Phoebe would just grow up and start acting her age; or better yet, thinking before she acts. But even though they've learned that they have some great destiny and magical powers, Phoebe continues to let her emotions guide her actions. So Prue continues to lecture and argue with her and wish she would grow up.
Then Prue dies.
And Phoebe does grow up. Just in her own way.