In chapter 6, Nick gets the idea to make the word frindle into a real word by having his friends use that word instead of the word pen. Nick and his friends go to the shop and they ask the woman behind the counter to sell them a frindle. The woman of course has no idea what they are talking about. She replies, "A what?" Nick then asks for "a black one," but the woman behind the counter still has no idea what he is asking for. She leans towards him and asks, "You want what?"
At this moment, Nick points towards a selection of ballpoint pens behind the woman and repeats, " A black one, please." Seeing what Nick is pointing toward, the woman understands that he is asking to buy a pen. Accordingly, she hands him a black pen.
In the following days, Nick sends five of his friends to the same store to ask the same woman for a frindle. These friends all leave the store with a pen. By the time the fifth friend, Janet, asks for a frindle, the woman behind the counter knows exactly what she means. The woman hands Janet a pen without having to clarify what exactly it is that she wants. In this way, Nick makes the word frindle "a real word."
Nick later gets his friends to agree never to use the word pen again, and to only ever use the word frindle in its place. All of his friends sign an oath "with Nick's frindle."