Models are representations of phenomena that are difficult to observe directly. For example, the progression of atomic models can explain how atoms are put together and what matter is made of. A theory explains natural phenomena based on observations, experiments, and fundamental principles. For example, Galileo dropped light and heavy objects from a tower. From this experiment, he proposed a theory that a falling body's acceleration is independent of its weight. However, the rate of fall of a feather and a baseball are not the same, unless they are dropped in a vacuum to eliminate air resistance. Galileo's prediction corresponds to an idealized model that applies only to objects with a weight much greater than the force exerted by the air. A scientific law is a concise statement describing the behavior of any natural phenomenon. A law predicts what happens, while a theory proposes why. For example, a law predicts how far a ball travels when launched at a certain angle, while a theory uses gravity to explain the parabolic trajectory of the ball.