Liquids also have a weight. The standard measurement is that 1 litre of pure (distilled) water weighs 1kg. For more viscous liquids, each millilitre will weigh more. This is why when I make my chocolate fudge cake and the recipe calls for 250ml of Maple syrup, I have to buy a 330gram bottle from the supermarket and use the vast majority if it
It could be for anything. How much milk do you feed a kitten? How little milk would it take to drown a person? How much milk is left in the cup?