Please see what this sentence means: there is a joke that the matchstick suddenly feels itchy, so it reaches out and scratches, scratching and scalding itself.

Meet your girlfriend and take the subway back together. Maybe she was exhausted by a day's work. She sat down for a long time and fell asleep on my shoulder. And all I can do is try to keep my body from shaking, and even hope that time will stop at this moment and let other passengers see what her saliva is like.