Milk is a valuable resource that is gained along with Achievements.
It is used along with some upgrades to increase cookie output and is gained at the rate of 4% per Achievement, excluding Shadow Achievements. As of version 1.0453, the total number of Achievements is that count towards milk is 148 and the maximum attainable Milk amount is 592%.
Kitten Upgrades (Kitten helpers, Kitten workers, Kitten engineers, Kitten overseers, and Kitten managers) increase cookie production based on the Milk percentage. As the Milk percentage increases it gains different names, although this does not change its effects on cookie production.
The cookie can also be dunked in the milk by zooming your browser in to roughly 200%, or resizing your browser window, earning you the Cookie-dunker achievement.
Below 100%, the Milk is plain white. Starting from 104%, it turns gradually into chocolate Milk, with the color turning brown. As it reaches 200%, it becomes pure chocolate Milk. Past 200%, it turns gradually into raspberry Milk. At 300%, it is pure raspberry Milk. Past 300%, it turns gradually into orange juice. As it reaches 400%, it becomes pure orange juice. After 400%, it turns again gradually into Caramel Milk. At 452%, it is pure Caramel Milk.
Below are five images the game uses to overlay the lower background.
TIP: The more milk you have, the more CpS the Kitten upgrades from the shop will get you.
At 52% Milk, you may unlock the Kitten helpers upgrade for 9,000,000 cookies. At 100% Milk, you may unlock the Kitten workers upgrade for 9,000,000,000 cookies. At 200% Milk, you may unlock the Kitten engineers upgrade for 9,000,000,000,000 cookies. At 300% Milk, you may unlock the Kitten overseers upgrade for 900,000,000,000,000 cookies. It was previously thought that a new Kitten upgrade should become available when the hundredth achievement was added to the game. However, it did not happen.
With Kitten helpers, you gain 5% of your Milk percentage as more cookies, stacked multiplicatively with cookie upgrades. If you had 100% Milk, Kitten helpers, and 2 cookie upgrades 5% each, your multiplier would be 115.5%
With Kitten workers, another 10% of your Milk is added to your multiplier. This is stacked multiplicatively with Kitten helpers as well as cookie upgrades.
Kitten engineers adds another 20% of your Milk into your multiplier. This is stacked multiplicatively with cookie upgrades as well as previous Kitten upgrades.
Kitten overseers adds yet another 20% of your Milk into your multiplier. Same rules as before.
Finally, Kitten managers adds yet another 20% of your Milk into your multiplier. Same rules as before. See CpS for details of the calculation.
Let's add several cookie upgrades 150% total, 300% milk and all four kittens: (665%+150%)*(100%+300%*5%) *(100%+300%*10%)*(100%+300%*20%)*(100%+300%*20%)=(665%+150%)*1.15*1.30*1.60*1.60=3119.168%