The Most Difficult to Grow

"Professor, what is the most difficult thing in the world?" Hearing this question, I turned back curiously to hear the answer. It started raining outside the car at some point, and it covered the window like cow hair. It was white and hazy. In a moment, the scenery outside the car was blurred. Professor Gu smiled and looked unusually relaxed: "The most difficult thing in the world is longevity. Things are like this, feelings are like this, and even life... Is like this."