A student created a model to explain half-life during alpha decay using 50 pennies and a plastic container with a lid. He began by placing all the pennies in the container with the heads side facing up. He closed the container and shook it five times. He opened the container and removed any pennies with the tails side facing up. He alternated shaking the container with removing the "tails" pennies until the container was empty. In this model, what do the pennies showing tails represent?