It is Saturday afternoon. Liu Tao and his friend David are taking a walk in the park. There are many public signs in it. They mean different things. Liu Tao knows a lot of things about them. Suddenly, they see a short man walking on the grass. Liu Tao points to the sign on the grass and says to him, "Sir, look at the sign! You can't walk on the grass. "The man smiles and says, "You are right, young man. But I'm the park keeper. I'm looking after the grass. "
