There are always some people who leave traces in a certain period of your life. However, with the passage of time and the helplessness of the world, they are separated and then forgotten, just like the long white mist trailing behind the tail of an airplane, slowly turning from clear to blurry, from blurry to disappearing. Maybe you will forget, but there are always some memories that linger and are sealed in the depths of memory. When you suddenly raise your head one day and look at the blue sky, you will suddenly remember that you once looked up at the blue sky like this. A plane flew across the sky lightly, leaving a long white mist. Melancholy, emotion, and thousands of emotions suddenly emerged, making your eyes wet without realizing it.