When two balls, which are falling at the same time, bounce of the base, total mechanical energy and total momentum of the balls are conserved. Bouncing occurs in two stages following in very short time period: the bottom ball bounces of the base and while moving up, collides with the smaller ball that is still falling. In this collision, upper ball, which has smaller mass, gains more energy at the expense of the bottom more massive ball – and in result it’s elevated to greater height.