open another bottle of milk when there is already one open.
fridge at work has three - 3 - THREE - open bottles of milk. (that;s three, not three hudnred and thirty three, although I wouldn't put it past them).
Why do people do this? just use one until it is empty then move on to the next one. preferably in expiry date order. if they are worried that the milk is going off, it's probably because they are opening all of them and making them wilt faster.