A husband and wife decided they needed to use “code” to indicate that they wanted to have sex without letting their children in on it.
They decided on the word Typewriter.

One day the husband told his five year old daughter, “Go tell your mommy that daddy needs to type a letter”.

The child told her mother what her dad said, and her mom responded, “Tell your daddy that he can’t type a letter right now cause there is a red ribbon in the typewriter.”
The child went back to tell her father what mommy said.

A few days later the mom told the daughter, “Tell daddy that he can type that letter now.”

The child told her father, returned to her mother and announced, “Daddy said never mind with the typewriter, he already wrote the letter by hand.