Here's a spacer's canteen filled with yinrih milk. Their milk and their ink have similar origins, which is why the milk is blue, and why it is sweated from the palms of the forepaws. It doesn't have the earthy rain smell that ink has.

The milk has very strong antimicrobial properties to compensate for the fact that it's secreted from a surface in constant contact with the ground. Milk production is stimulated by exposing the palms to saliva. Kits lick their dams' paws to nurse. The “on-demand” nature of milk production is an adaptation to prevent dams from losing their grip when climbing, since constantly sweating milk or sweating based on a cycle would make for slippery paws at inopportune times.

Hearthkeepers collect their own milk by licking their paws and soaking the milk up with a sponge, then wringing the milk out into a container. The milk is then blessed and used like holy water. Hearthkeepers often sprinkle blessed milk via an aspergillum held in the tail.

The phrase “By the palms that nursed me!” means something like “oh wow” or “holy crap”.