Misunderstanding
 

Dave is at home doing his 2nd grade arithmetic homework when his mother walks in ans hears him say,"One plus one, the son-of-a-bitch is two. Two plus two, the son-of-a-bitch is four."
His mother says, "Where did you learn that?"
He replies,"from Sister Amalia Marie." Shocked, she goes to school with him the next day and asks Sister Amalia Marie why she is teaching her son foul language. The nun asks where did she ever come up with that idea. Dave's mom repeats what she had heard him say,"One plus one, the son-of-a-bitch is two. Two plus two, the son-of-a-bitch is four."
Sister Amalia Marie answers, "He completely misunderstood me. What I said was 'One plus one, the sum of which is two. Two plus two, the sum of which is four.'"

Now that's a groaner!