填空题

Mrs. Black has two sons. They are very strong but very lazy. Whenever Mrs. Black asks them to do something, they always find some excuses"Mom, I am busy on my homework." or"I am suffering from a headache." One day, Mrs. Black took back a picture with a washing-machine on which there was a girl standing by the machine. She said to her sons:"There is a newlydeveloped washing-machine. It can make our housework simpler. Would you like to buy one" "Yes," the two sons said without any hesitation."Buy what’"’The machine and the girl," said the two sons at the same time."Then she can wash our dirty clothes for us," one of them explained.

Why did the two sons want to buy the girl（）