If I could keep any app on the phone I would probably keep the Reparo app. It sounds like it could fix anything. I normally break a lot of stuff so this would help me fix it. So in class if I broke my pencil or my paper ripped and I was too lazy to get another one or sharpen my pencil I could just use the Reparo app to fix my paper and sharpen my pencil. Or if I ever cracked the screen on the phone I could just use the Reparo app. I think the Reparo app is the most useful app on the phone.
