Pen.

Imagine.
        One day, you lost your pen. When you lost your pen, you have no pen. When you have no pen, it means there is no notes. When there are no notes, it falls to no study done. Since no study is done, you'll fail your exams. Exams failed equals to no degree obtained. Without a degree, you have no work. When you have no work, subsequently you'll have no money. When there is no money, you'll get no food to eat. Now, no food means you get skinny. When you are skinny, you are ugly. Ugly leads to no love, and when there is no love you won't get married. No marriage causes you to have no children, which then leaves you alone without company. When you are alone, you get depressed. Depression leads to sickness, and when you are sick enough, you are qualified to be dead and you are lastly dead of course.

Lesson : Don't lose your pen, you will die.