While witnessing the duke and the king try to scam unsuspecting people, Huck describes the duke using a simile:

The duke he never let on he suspicioned what was up, but just went a goo-gooing around, happy and satisfied, like a jug that’s googling out buttermilk.

Here, Huck compares the duke to “a jug that’s googling out buttermilk” as a way of amplifying the duke’s feigned naivety. This is an important moment for Huck because it demonstrates his awareness of the duke’s capacity to be fraudulent, as well as his disdain for it. Whereas Huck trusted both the duke and the king at first, he has matured and become more discerning, seeing through the ways that they lie and scheme to steal people’s money. This is all part of Huck’s process of growing up over the course of the novel, moving from an innocent child to a young man with a moral compass of his own. This simile helps clarify how he sees the duke and also hints at how he sees himself—as someone who would never willfully deceive people like this.