A mother and her young son get onto a bus in a city and sit down. The bus conductor comes to them for their money. The mother says ,"I want one ticket (票) to the zoo." Then she gives the conductor the money. The conductor looks at the small boy for a few seconds and says to him, "How old are you, young man?" The mother wants to say something, but the conductor stops her, and the boy says, "I'm four years old at home, but two in buses." The mother's face turns red. She has to give the money of another ticket to the conductor.