mayonaise.

...

 there's this guy that plans to have sex with his girlfriend, they can't go to her house `cause her parents are having a house party;; his little brother & him share a room & more-- a bunk bed! so, they decide, since they're horny, that they'll just do that... & he sleeps on the top. they couldn't make his 8 year old brother know that they were having sex... so...

 guy: when you want it harder say lettuce, when you want it faster say tomato...

so, they have sex, & all through the night while the little brother is sleeping the girl keeps saying: "lettuce" "tomato" "lettuce" tomato"...

the little brother wakes up & says:

"can you guys stop making sandwiches up there? you're getting mayonaise all over my face."