Which is greener: Glass bottles, plastic bottles, or aluminum cans?

Hot enough out there? Bet you're thirsty.

Once you choose what you want to drink, there's another big decision: What are you going to drink it from?

The container — and I'm not talking about the reusable water bottle you always carry — is a major part of your beverage footprint. (Would that be your "drinkprint?")

The good news is that companies are responding to greener consumers and are making huge sustainability strides. Just by drinking in 2012 instead of 1982, you're already to the good.

Glass bottles, for instance, are 40 percent lighter today than they were 20 years ago, which means it takes less fuel and produces fewer emissions to transport them.

Ditto plastic. A few years ago an empty half-liter water bottle weighed 22 grams. Now, it's 8.5. (Soda bottles are heavier so they can withstand the carbonation.)

As for cans, today's models have a carbon footprint 43 percent lower than those in 1993.

But you still want to make the best choice. Which is it?

In the absence of an endlessly patient academic willing to spend untold hours on an independent life-cycle analysis, I issued a challenge to the three industries.

"This is your moment," I said. "Tell me why your containers are more sustainable than the others."

Ladies first, so we'll start with Lynn Bragg, president of the Glass Packaging Institute.

First on her list of eco benefits is that a glass bottle can be recycled endlessly into other glass bottles. Once in the recycling bin, "we can get it back to the consumer as another glass bottle in 30 days."

Adding recycled glass to the mix means manufacturers' furnaces can run at lower temperatures. And for every six tons of recycled glass used, the carbon dioxide emissions drop a ton.

When manufacturers do have to start from scratch, the raw materials are readily available in the United States.

Glass doesn't have the estrogen-mimicking chemical bisphenol A, which sets it apart from cans and plastic.

But, oof, it's heavy, so transportation is environmentally expensive. And, yes, glass breaks. So you might get some waste there.

Plastic bottles have a big target on them from environmental groups because they're made with petroleum.

So weight is where plastic bottles win out, said Dennis Sabourin, executive director of the National Association for PET Container Resources — PET being the short version of the plastic resin polyethylene terephthalate.

Check out the shapes, too. Many bottles are being made with shorter necks, which means that when it comes to shipping, you can get more bottles in a smaller space.

If no additives are used, PET bottles can be recycled back into PET bottles, Sabourin said. Otherwise, they get "downcycled" into carpeting, clothing, and other fibers.

If waste of the contents is a concern, most plastic bottles have caps that can be screwed back on.

Now for aluminum, and a few words from Steve Gardner, spokesman for the Aluminum Association.

Like plastic, aluminum cans are lightweight — so they have a smaller transportation footprint. And because they stack tightly, there's less wasted space when they're shipped.

All the containers keep out air, but aluminum also keeps out light, which can degrade the product, leading to waste.

Like bottles, cans are endlessly recyclable, can into can into can. Plus, making a can from an old can instead of the raw material uses five percent of the energy and generates five percent of the emissions. (Although, when new material is needed, it's bauxite that is mined overseas, with most of it coming from Australia.)

All three beverage manufacturers touted their products' recycling rates, as if it had to do with the product instead of user attitude. In the case of aluminum, there's something to that claim of superiority for recycling: As a commodity, aluminum brings a significantly higher price, so much of the cost of a recycling program is actually paid for by the cans.

So perhaps not surprisingly, a lot of aluminum is recycled. Today's typical can contains 68 percent recycled content.

You could also say aluminum helps reduce litter: It's valuable enough that someone inevitably picks it up.

But the lining of aluminum cans contains bisphenol A.

The complexity of the "drinkprint" calculation seems to be increasing.

Craft brewers — such as Sly Fox, based in Pottstown — are starting to can their beers instead of bottling them. For them, the cans work out to be more eco-friendly, said SF brewmaster Brian O'Reilly. They're also cheaper. And you can take a can places where bottles aren't allowed.

Yards in Philadelphia still uses bottles, but mainly because it's too expensive to buy an entire second system, said president Tom Kehoe. He's mulling it, though.

Some drinks are starting to reappear in glass. As part of the company's overall effort to diversify portion sizes, some Coca-Cola products are going into eight-ounce bottles.

And a whole new category of containers is in the chute: bottles from plants.

Coca-Cola now makes a "PlantBottle" from 30 percent plants. Although some enviro groups didn't like that the plants were sugarcane coming all the way from Brazil, a company spokeswoman said the crop is "rain-fed" on "abundant arable land" (in other words, no rain forests were felled) and fertilized with organics. Coca-Cola has joined with four other companies to accelerate development of a bottle made totally from plants.

Not to throw my hands up, but I really don't want to be a container cop here. Clearly, there's no one best choice for every person or every situation.

But selection can reflect your personal concerns.

Want to encourage more recycling? Go for the can that will help fund it.

Distrust bisphenol A? Go for the bottle.

If you do go for the bottle, go for local contents, such as beer from Philly instead of the Rockies, to lessen the shipping footprint. Conversely, if you have to get something from far away, get it in a can or in plastic.

Whatever the container, the overriding message is to recycle it. Every industry has reams of data showing how much smaller its footprint is with greater recycling.

Meanwhile, all this has worn me out.

I think I'll go have a beer. From a keg.