hands
Explanation
This comic is a joke about a specific and absurd intimate etiquette rule.
The comic takes place entirely in darkness (black panels with white text). Someone exclaims, "Oh my God, what are you doing?" and asks, "...Boob grab?" The other responds, "You're using a forbidden method!" The rule is then explained: "You can grab ONE boob in ONE hand or BOTH boobs using TWO hands. You can even attempt to grab TWO boobs in ONE hand. But you can't grab ONE boob with TWO hands!" The other person compares this to "getting chewed on by a giant duck beak" and quacks.
The humor comes from treating an intimate act as though it has rigid, codified rules like a board game or sport, with certain moves being "forbidden methods." The specific forbidden combination (one boob, two hands) is funny because it is the one arrangement that does sound physically awkward, and the comparison to a duck beak captures why -- two hands closing on one object does resemble a beak. The all-black panels suggest this is happening in the dark, adding to the fumbling comedy of the situation.