Parents playing with children on one hand, chatting about some droll table tennis rules on the other hand, could not only teach them more knowledge, but also have more fun. Do you know how to judge the twelve situations below?1. While playing, suddenly you switch to use the left hand holding the racket to return. Is it valid?2. While returning, you throw out the racket and it successfully hits back in the air. Is it valid?3. Your opponent’s stroke has hit your half table already, but as it .....