Although it is heavy, I can move a table in my classroom. To minimize the possibility that I might hurt myself, I ask a student to help me move the table. This student is very helpful as she contributes a great deal to the effort. However, we move the table too far. I ask a second student to give us a hand. The second student contributes to our task but his contribution is less than that of the first student. We add a third student to our endeavor. His contribution is smaller still. At some point, the last student added to the task, moving the table, will get in the way and cause us to drop the table…his contribution will be negative.