A lot of people buy their milk in plastic jugs.
But there's not much of a market for recycling plastic.
Researchers have looked at what's been used in the past, including paperboard cartons and refillable glass bottles.
The University of Michigan's Center for Sustainable Systems research shows glass bottles were being refilled at least eight times.
Then, they wouldn't be returned or they would break.
But glass bottles are heavy and use more energy to transport.
A more sustainable model for milk might be using refillable bottles made from a more durable plastic.
The researchers say even more important is avoiding wasting milk because it has a larger environmental footprint than its containers.