Mrs. Jones is waiting for an important telephone call, but she has no vegetables at home, so she leaves the baby and says to her five-year-old son, "I'm going to the shops, Jimmy, and I'll be back in a few minutes." While she is out, the telephone rings, and Jimmy answers. "Hello," says a man. "Is your mother here?" "No." answers Jimmy. "Well, when she comes back, tell her Mr. Baker telephones." "What?" says Jimmy. "Mr. Baker, write it down B-A-K-E-R." "How do you make a B," asks Jimmy. "How do I make B? Listen, little boy, is there anyone else with you? Any brothers or sisters?" "My brother Billy is here." "Good, I want to talk to him, please." "All right." Jimmy takes the telephone to the baby's bed and gives it to Billy. When their mother comes back, she asks, "Does anyone telephone?" "Yes," says Jimmy, "a man. But he only wants to talk to Billy." Billy is   _  .
Jimmy's brother Mrs. Jones's brother Mr. Baker's son Jimmy's sister