"The Letter" joke

When Kevin came home, his wife, Liz, was crying.

"Your mother insulted me," Liz sobbed.

"My mother? How could she do that when she is on vacation on the other side of the world?" Kevin asked.

"I know. But this morning a letter addressed to you arrived. I opened it because I was curious."

"And?"

"At the end of the letter it said,' PS. Dear Liz, when you have finished reading this letter, don't forget to give it to my son.'"