skeptical
Explanation
The Joke
A parent is advising their child not to tell other kids that Santa is real, warning that eventually they will find out the truth, realize they were lied to, and then become skeptical of everything. The parent frames this as a cautionary tale about the dangers of deception. In the final panel, the punchline arrives: the child points out that Santa IS real, and so are the Easter Bunny, the Tooth Fairy, and the Needle Elves. The parent sheepishly agrees, admitting the family dog can talk "because he's the Lord High Inquisitor of Snarfblatt."
The comic sets up the audience to expect a standard "should we lie to kids about Santa?" debate, then subverts it by revealing that in this universe, all of these fantastical creatures actually exist. The parent's original concern about kids becoming skeptical is rendered absurd because there is nothing to be skeptical about.
The Humor
The joke works on multiple levels. First, it subverts the familiar real-world parenting dilemma about Santa Claus by placing it in a world where the supernatural is mundanely real. Second, the escalation from Santa to increasingly absurd creatures (culminating in "Needle Elves" and a talking dog who holds a bureaucratic fantasy title) heightens the comedy. The comic also slyly comments on skepticism itself -- suggesting that in a world where everything improbable turned out to be true, skepticism would indeed be unwarranted, flipping the usual pro-skepticism message on its head.