Little Mary goes up to her mom and says "Daddy said to tell you he needs to use your typewriter to write a letter...."

Mom smiles because she knows this is their secret code for sex. Unfortunately since she was on her period, she told little Mary: "Tell your daddy he can't use it because it only has red ink right now"

The next day, mom says: "Go tell daddy he can use my typewriter now". Little Mary runs off, and returns a minute later and tells her mom: "Daddy said it's too late because he already did the letter by hand"