A challenge is a request to review a line call. Players use it when they disagree with a decision. Each player has a limited number of challenges. If the review shows the call was wrong the point changes. If the call was correct the player loses a challenge. Challenges add fairness to close moments. Players must decide quickly whether to use one. For example a serve may appear out by a small margin. The player challenges the call immediately. The review then confirms the correct decision. Smart use of challenges can shift momentum. Poor timing can waste valuable opportunities.