Making Sandwiches

After going out with his girlfriend for a month, Dave decided he
wanted to sleep with her. Unfortunately he still had to share a
room with his 9 year old brother Jonny. One night, he waited for
Jonny to fall asleep then he rang his girlfriend and asked her
to come over. Quietly they climbed onto the top bunk of the
boys' bed and started to get undressed. After a few moments,
Dave realised that they would have to make up some sort of code
for what they wanted each other to do if they didn't want Jonny
to hear them. Eventually they settled for "Lettuce" meaning
Harder, and "Tomato" meaning Change Position. Soon they began to
get into a rhythm.
LETTUCE!
TOMATO!
LETTUCE!
LETTUCE!
TOMATO!
OH WAIT! PULL IT OUT, PULL IT OUT!
I CAN'T GET PREGNANT!
Then suddenly quick as a flash, Jonny sat up and screamed at
them, "Can you two PLEASE stop making sandwiches?! You're
getting mayonaise all over my face!"