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 1.0393, the total number of Achievements is 109, but the maximum attainable Milk amount is 388%
Kitten Upgrades (Kitten helpers, Kitten workers, Kitten engineers and Kitten overseers) 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 their browser in to roughly 200, or resizing their browser window, earning them the Cookie-dunker achievement.
Your Milk percentage can be seen on your Stats under Achievements.
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% to 388%, it turns gradually into orange juice.
Below are four images the game uses to overlay the lower background.
TIP: The more milk you have, the more CpS the Cat upgrade 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. If Orteil follows with current patterns, a new Kitten upgrade should be added when there are 100 achievements, enough to get 400% milk.
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.
Finally, Kitten overseers 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%