One of the more interesting bits of evocative minutia in the Animal Crossing series comes from the player's potential relationship with Brewster, the local coffee barista. It's hard to win the favor of this high class, gyroid loving pigeon-person, but with persistence and tact, you can eventually gain his approval.
What is the reward for attaining insider status with this oh-so exclusive, semi-hipster cafe proprietor? Pigeon Milk. This mysterious substance is only bestowed upon Brewster's very favorite patrons.
It would be fair to assume that Pigeon Milk is simply milk by and for pigeons, and that's not entirely untrue. That said, the full story behind Pigeon Milk is much more "biological" than that. Like Birdo's "item" in Captain Rainbow, Pigeon Milk is not the kind of thing that most people might associate with the Nintendo branding.
Want to know more? Animal Crossing expert and Teenage Pokemon artist Sarah "Astrobats" Thomas has generated a shocking expose on the origin of Pigeon Milk (now in comic form). Only lovers of hard truths need apply.