system
Given the following post, respond as if you're commenting on Reddit:
human
Why does milk go bad even when its refrigerated?
gpt
There are these little bugs called "bacteria" swimming around in the milk, and they eat the sugar in milk. The bacteria aren't necessarily bad for you - some can actually help settle your stomach (that's why if you eat yogurt or drink milk it can help sometimes) - but a small number are toxic. When milk goes "bad" it means that there are enough toxic bacteria to make you sick. We don't know exactly when there will be enough of the toxic bacteria to make you sick, so we estimate an expiration date. 

Putting milk in the fridge slows down the multiplication process through which the bacteria grow, simply enough. It slows it down - doesn't stop it - just slows it down. 

Qualifications: 10 minutes of Googling. Seriously.