You get in your car to drive to the store (1). you turn the key and the engine starts making a clicking sound, but does not start (2). you think to yourself, "the battery must be dead" (3a), so you go into your neighbor's garage and borrow the battery from your neighbor's car (with permission, of course) and exchange it for the battery in your car (3b). you also put your presumably dead battery in your neighbor's car (4). you figure that if the battery in your car is dead and you replace it with a working battery, then the car will start and your neighbor's car won't start (5). you get in the car again, turn the key, and your car starts right up (6). your neighbor's car does not start (7). you replicate this experiment with 10 sets of cars and batteries and get the same result each time (8). which part is an experiment?