answer:I asked my darling husband, who’s an avid tennis player and fan. Paul says that there’s really no science to it. All three balls look and feel the same. If you’ve ever been to a professional tennis tournament, you’ve seen that a ball knocked into the stands gets retrieved. That’s because all 3 balls must have the same wear and “bounciness,” and all 3 are retired and replaced at the same time. So, when a player tosses the ball to the ballboy/ballgirl, there really isn’t an evaluation process. Keep two; toss one; play.