In the short story, Boys and Girls, what joke did the narrator's father make about her to the feed salesman? What was the narrators reaction?

From the text:

One time a feed salesman came down into the pens to talk to him and my father said, "Like to have you meet my new hired hand." I turned away and raked furiously, red in the face with pleasure.
"Could of fooled me," said the salesman. "I thought it was only a girl."