It's that "click". The sound of "click" like you insert your key to your door and turn it to unlock the door. The sound of "click" when things seems to work just how you want it. The "click" when you pull down that switch on the lamp on the ceiling and suddenly the darkness become so bright and you can see everything, clearly. I need to find that "click". Now, where's my glass of white wine.
PS: may be that's why key in French is la clé sounds like "click"