The conservation law of energy and momentum can be examined and demonstrated by a well-known collision experiment. In this experiment, several identical elastic balls are suspended from a horizontal frame. When the ball at one end is pulled aside and released, thus allowing it to swing like a pendulum, it hits the next ball. The outcome is fascinating: the ball at the farthest end is knocked away from the others with nearly the same speed that the first ball had initially, and all the other balls remain almost at rest. Why does this happen? Why is this the only outcome that occurs? Why not others? What happens if we change the size or mass of the ball? In the current essay, the authors describe this principle and provide examples where students can apply it. For each problem, the authors provide both the force consideration solution approach and the energy consideration solution approach.