1) "The ITF Play and Stay campaign aims to increase tennis participation worldwide, by improving the way starter players are introduced to the game. The campaign promotes the use of slower red, orange and green balls that give players more time and control so that they can serve, rally and score (play the game) from the first lesson. By using slower balls the starter players have more time and more control to make the game more fun for them at the introductory stage. The ITF Intro to Tennis Task Force recommends the red, orange, green progression for starter players. This progression focuses on a range of slower balls and court sizes to introduce the game effectively to both adults and children. Using these slower balls will help the players to develop the most efficient technique and to be able to implement tactical situations in matches that, in most cases, could not be performed using the normal ball on a full court. The ITF recommends that, except for exceptional players, all players aged 10 and under should use a slower red, orange or green ball in training and competition." Source and further information: http://en.wikipedia.org/wiki/Tennis_ball 2) If your daughter is confused about changing all the time the kind of balls during training, you could give this feedback to the trainer, maybe they could take this into account. Those different kinds of balls have not always existed.