When playing a game of 21 questions with a girl, one should ask questions that will provide information about her. This is especially helpful and important if the girl is someone who is a romantic interest or someone with whom you want to form a relationship.
Start the game by asking a silly question such as, "What is your favorite color?" This is a safe starter question that can easily lead into more discussion. Continue the game with a question such as, "Where are you from?" This will get her talking about her past and allow you to understand more about her background. "Do you have any siblings?" is also a good question because you are showing an interest in her family while also learning about how likely she is to share or understand the opposite sex. The more you learn about her background and her family, the more sensitive you seem and the more you know about her personality and tendencies. Gradually work your way into deeper questions such as, "how long was your longest relationship?" This will start a conversation about past relationships and reasons why they didn't work out. You will also get a feel for how likely she is to enter a committed relationship. Once the game has continued for a while and you are becoming more comfortable with each other, you can ask questions that spark more interesting conversation such as, "What do you look for in a boyfriend?" or "What do you like about me?"