A joke about pencils

Once in art class, the monitor didn't bring a pencil, so he asked me to borrow a 4B pencil. I only had a 2B pencil and gave it to him. Then he saw it and said, "Don't you have 4B?" I said, "Yeah." He said, "2B is 2B." Then the teacher came in and shouted, "class." The monitor was probably still thinking about what happened just now, so he shouted "2B". The teacher was shocked: "Find a cigarette, who is 2B?" The monitor was shocked by the war for a long time and said, "I said the pencil is only 2B, not 4B." The teacher smiled.