Cats can drink milk when they are kittens because they produce the enzyme lactase, which helps them digest lactose in their mother's milk. However, as cats mature, most lose the ability to produce enough lactase, making them lactose intolerant. This intolerance means that drinking milk, especially cow's milk, can cause gastrointestinal upset, including diarrhea and abdominal pain. Therefore, milk is generally not recommended for adult cats as a regular part of their diet. Although some adult cats may tolerate milk in small amounts, it is not a nutritious or necessary food for them and can contribute to obesity and nutritional imbalances if given frequently. For hydration, cats should be provided with clean water rather than milk. In summary, while kittens drink milk naturally, most adult cats should avoid cow's milk due to lactose intolerance and potential health problems. If milk is offered occasionally, it should be lactose-free or specially formulated cat milk to minimize digestive issues.