meant business

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."