Answer:
I am round. I have only one line. Circle is not my name indeed. What am I?
Who keeps the ocean clean?
I am an instrument you can hear but cannot touch or see. What am I?
The letter S is added to pluralise most words in the English language. eg. Letter to Letters Word to Words and Language to Languages. But I am a word that is pluralised with the addition of the letter C.
I have thirteen hearts but no body or soul.
No matter how little or how much you use me, you change me every month.