Change mind about learning math joke

A mom and dad were worried about their son not wanting to learn math at the school he was in, so they decided to send him to a Catholic school. After the first day of school, their son comes racing into the house, goes straight into his room and slams the door shut. Mom and dad are a little worried about this and go to his room to see if he is okay. They find him sitting at his desk doing his homework. The boy keeps doing that for the rest of the year. At the end of the year the son brings home his report card and gives it to his mom and dad. Looking at it they see under math an A+.

Mom and dad are very happy and ask the son, "What changed your mind about learning math?"

The son looked at mom and dad and said, "Well, on the first day when I walked into the classroom, I saw a guy nailed to the plus sign at the back of the room behind the teacher's desk and I knew they meant business."