“Having found the feather...”

So the young men, having found the feather, took it out with great glee; and looking around for something to replace it, they espied in a corner of the room some pieces of coal, wherewith they filled the casket; which they then closed, and having set the room in order exactly as they had found it, they quitted it unperceived, and hid them merrily off with the feather, and posted themselves where they might hear what Fra Cipolla would say when he found the coals in its stead

In the tenth and final novella of the sixth day narrated by Dioneo, one character presents coals and a feather as something that they are not. To understand the role of the objects, let us first retell the novella.