A very smart three-year-old child is given a wagon for her birthday. she refuses to use it. "after all," she says, "newton's third law says that no matter how hard i pull, the wagon will exert an equal but opposite force on me.

The reply would be, "The force that you apply on the wagon will cause it to move forward if it is bigger than all opposing forces on the wagon. You cannot relate Newton’s third law to the forces on a single object. The law only relates to forces acting on two different objects. These can never add to zero net force because they act on different objects."