A situation like this maybe:
Someone gets a bike. They ride it and all, and then all of a sudden they fall down and get hurt
If you only knew the first part on how the person got a bike, when you write a response, you would only write about that part, but getting hurt after getting the bike is also a huge part of the "story". You could write like how you were so happy when you got your first bike, but you wouldn't know to write about the time you got hurt when you were riding down a hill or something.
Hope this helps!
