"Sandwiches" joke

One day two kids parents decided that they wanted to go on a vacation, so the youngest brother was forced to live with his older brother at college.
When the little bro showed up, the oldest brother said that it was reallly cool that he is staying here, and said that there is only one rule.
IF I COME HOME WITH A GIRL, I GET THE TOP BUNK, AND YOU HAVE TO GO STRAIGHT TO BED, AND GO TO SLEEP!
"Okay,"
"Okay,"
Later that night, the little brother, watching TV, heard his brother coming down the hall, but he heard a girls voice with him, so he did what he was told and went straight to bed.
Faking sleep, he watched the older brother and his new love Tina, go up to the top bunk.
"Tina, anytime you want me to go harder, say lettuce, and anytime you want me to change positions, say tomato."
"Haha, okay" replied Tina.
So, in the middle of the night, the little brother is awakened by loud screams of the words lettuce and tomato
"Lettuce lettuce tomato lettuce lettuce lettuce lettuce tomato."
The little brother, fed up, reaches up to the top bunk and screams, "Will you guys stop making sandwiches up there? You are getting mayonnaise all over me!"