This guy is nearing the end of his senior year in highschool, unfortunately he still has to share a room with his younger brother who is about 5 or 6.
One night, he decides to bring his girlfriend home for a little fun. They have bunk beds and the guy notices that his younger brother is already asleep on the lower bunk, so he and his girl climb up to the top bunk.
As you might expect, things start to heat up. Remembering that the guy's little brother is sleeping below, he tells his girlfriend to whisper, "lettuce" if she wants it harder and "tomato" if she wants a new position.
"Lettuce, tomato, lettuce, tomato, lettuce, lettuce." it sounds.
Then the little brother chimes in, "Hey, would you guys stop making sandwiches up there? You're getting mayonaise all over my face."