A imaginary worm made to distract/get rid of people you dislike.
Annoying Boy: Hey girl. Want to hang out later tonight?
Girl: Only when you find a sand worm and show it to me.
Annoying Boy: YESSS! *runs to look for sand worm*
Girls friend: huh? why did you tell him that?
Girl: I guess he doesn't know that sand worms don't exist ;)