Milk is a resource in Cookie Clicker that unlocks the Kitten series of upgrades, which greatly increase cookie output rate in direct correlation to the amount of milk. Milk itself has no effect on the game beyond unlocking those upgrades. The percentage of milk you have is based on the number of achievements you have earned, with each achievement giving you a 4% milk allotment. The highest milk percentage you can achieve is 2072% with 518 achievements until new ones are added to the game.

To get more milk in Cookie Clicker, you need to unlock achievements. The best way to do this is to click a lot and buy CpS upgrades. Some easy achievements include "Making some dough," "Hardcore baking," "Steady tasty stream," and "Clicktastic".

One exciting thing about milk in Cookie Clicker is that you get many different flavors as your percentage increases. However, these flavors dont affect the game and are just a fun way to see your upgrades progress.

In summary, milk in Cookie Clicker unlocks the Kitten upgrades, which increase cookie output rate. The percentage of milk you have is based on the number of achievements you have earned, and you can get more milk by unlocking achievements. The different milk flavors are just for fun and dont affect the game.