"When you go back to your Mom's tonight, give her this envelope and tell her that since you are now 18, this is the last check she'll ever see from me for child support. Then, stand back and watch the expression on her face."
... Later ...
"Mom, Dad asked me to give you this envelope. He said to tell you that since I'm now 18, this is the last child support payment he'll ever have to make to you. Now I'm supposed to stand back and watch the expression on your face."
"Next time you visit your father, tell him that after 18 years I have decided to inform him that he's not your father. Then, stand back and watch the expression on his face."