It's a small yellow tennis ball. Tennis is a sport played by 2 people in singles or 4 people in doubles. Competitors use a stringed racket to hit a small rubber ball over a net in the opponent's court. Until 1972, the yellow balls we know today were any other color, although white and black were the predominant colors, as they were covered with wool, the natural color of which was that. After the introduction of color television, however, the situation radically changes. The whites start to merge too much with the line, and the blacks are too dark and the spectators were unable to follow the movement of the ball. Therefore, the heads of the world tennis federation sat down at the table and eventually it was decided that the color yellow would be used from that moment on. The reason is that the current yellow color of the balls is also the most visible to the human eye and does not strain the optic nerve as much. This also explains why white balls, which are still allowed in professional tennis, are so rare, and in 99% of cases, they are used on yellow balls.