I'm a native french speaker and only recently realized that "Qu'est-ce que c'est ?" ("what is it ?") literally translates to "what is it that it is ?".
Similarly you have "les choses que l'on voit" and "les choses qu'on voit" with the "l'" being there for prettyness.
All of these work and sound more natural in spoken language if you flip and say C'est quoi ? C'est qui ? C'est quand ? etc.