Henry was doing math homework

Henry was doing math homework, saying to himself…

“2+5, the son of a  is 7”

“3+6, the son of a  is 9”

His mother heard this and asked, “Henry! What is this nonsense you are doing?”

“Oh, Mom. I am just doing my math homework!”

Mom: “Is this how your teacher taught you?”

Henry: “Yes, Mom.”

Infuriated, she picks up her phone and calls the teacher.

Mom: “Are you teaching math to children by saying… 2+2, the son of a  is 4?”

There was silence for a moment. Then the teacher started laughing.

Teacher: “What I taught them was… 2+2 THE SUM OF WHICH IS 4.”