monster
Explanation
The Joke
A child tells his father there is a monster under his bed. The father, instead of simply checking, launches into a pedantic physics lecture. He argues that the universe does not really support the concept of "under" -- that it is a "clumsy concept" in the context of a transitional era where we think about "the particular rock we happen to live on." The monster, initially terrifying, starts begging for mercy as the father drones on. The father then triumphantly points out that he "clearly meant that it was in the manifold space part of his bed."
The dad then reflects that "good overprotective parents" should use situations like this to teach a lesson, and decides to explain about "the relative position of things." The monster, defeated not by bravery but by sheer boredom, mutters "I should've let it eat me." The child is shown as exhausted and miserable.
The Humor
The comic works on multiple levels. First, it subverts the classic "monster under the bed" scenario -- instead of the father being brave or dismissive, he is insufferably pedantic. Second, it satirizes a certain type of intellectually overbearing parent who turns every moment into a lecture, even when their child is genuinely frightened. The monster begging for mercy is a wonderful reversal: the creature of nightmare is itself terrorized by the father's relentless pontificating. The final panel, where the father decides to keep going and the child wishes for death, drives home that this kind of parenting is its own form of monstrousness.
References
The father's arguments loosely reference real concepts from physics and philosophy of science -- the idea that spatial concepts like "up" and "under" are relative to a gravitational frame of reference and are not fundamental properties of the universe. This is the kind of technically-true-but-socially-unbearable observation that characterizes the "well actually" personality type that SMBC frequently lampoons.