This is according to Newton's third law of motion states that for every action, there is an equal and opposite reaction. When the ball hits the floor, it exerts a force downwards on the floor (action). In response, the floor exerts an equal and opposite force upwards on the ball (reaction). This upward force causes the ball to accelerate upwards and bounce. While elasticity of the ball is important for the bounce to occur efficiently (it allows the ball to deform and then spring back), the fundamental principle governing the interaction force is Newton's third law.