Mayonaise
There were 2 brothers that shared a room. One was 18 and the other was 8. The 18 year old brought home his girlfriend at midnight. The little brother was sleeping so they climbed quietly to the top bunk. Things started getting hot and heavy so the guy told his girlfriend if she wants a new position to say "lettuce" and if she wants him to stop say "tomatoes".
So she started saying "lettuce, tomatoes, lettuce, tomatoes.... take it out I don't want to get pregnant." The little boy woke up and screamed, "Stop making sandwiches up their, your getting mayonaise all over my face!