Here is a quick riddle.
We are twins. Between us something smells. Can you guess who we are??
Please place your answers in spoiler tags.
Buttcheeks, because.. I don't explain it now :D
The word "between" has a couple of meanings - shyos's answer is the usual meaning, but there's also this option:
Two nostrils - between them, they make up a nose, which smells.