White balls are in continuous motion, they dont stop and move on the billiard table with minimal speed. When the red cue ball hits some white ball, it receives the impulse speed and turns yellow color. All touched balls turn yellow, while they have increased speed. While the ball is yellow (touched), it may get to pocket. But when the ball speed decreases to the minimal speed, the ball turns back white. White balls can not get to pockets, they just move slowly by the table. So, to put some ball to the pocket, the player must hit some white ball, and while it is turned yellow it may get to the pocket.