For whom is the half ticket?

A lady with her son and daughter came to the railway booking-office. She asked for two and a half tickets.

"For whom is the half ticket?" asked the clerk.
"For my son, of course," answered the lady.
"But he can't travel with a half ticket because he is wearing long trousers," the clerk said.
"Oh, is that how you judge?" the lady asked quietly.
"Yes, it is," the clerk answered.

The lady decided to teach him a lesson2 and said:
"Then I shall tell you, young man, that I can use the half ticket, and my daughter can travel free."
After this the clerk gave out the tickets.