Mistake(?) at "Il y a un fil qui ne sert à rien."
The right answer was supposedly:
There is a thread that is not useless.
That's one negation to many I guess. Shouldn't it be:
There is a thread that serves for nothing.
There is a thread that serves this question at http://forum.wordreference.com/showthread.php?t=558464. Well, in relation to 'ne sert à rien'. The example sentence there refers to an action (crying), not an object. And there is a song by Elisa Tovati - Ça ne sert à rien d'aimer - again referring to an abstract thing, love. BTW, I would go for 'thread that serves no purpose' over 'thread that serves for nothing'.