Paper or plastic? At grocery store checkout lines around the country, it's been part of the routine for decades.
But in San Francisco, Ross Mirkarimi, a member of the city's board of supervisors, says it needs to change.
And to him, it's not just a local issue -- it's a stab at protecting the world environment.
"You're talking about 12 million barrels of oil that are used nationally to produce 30 billion plastic bags in the United States," he told ABC News. "It's up to local governments to not wait for the federal government to get its act together."*
The plastic bags you get at the grocery store are convenient, strong and lightweight. But to environmental advocates, they spell trouble.
On a local level, plastic is less recyclable than paper or other materials. Recycle a paper bag and you can make more paper bags. Recycle an aluminum can and you can make more cans. But even if plastic is labeled as recyclable, it cannot be turned back into the same kind of material. Plastic from a grocery bag can be processed into parts for a park bench or other hard material, but not the light, flexible material that would make another bag.
The larger issue, though, says Mirkarimi, is that plastic comes from petroleum. Companies drill for it, spoiling distant parts of the world. Air pollution is generated when it's processed into plastic. And oil's principal use, as fuel, is pumping heat-trapping carbon dioxide into the atmosphere.
"We all have a responsibility in dealing with what I think is going to be an unabated oil crisis, an energy crisis," Mirkarimi said, "and I think our determination to save this planet, environmentally and economically, starts here at home."
In 2005 he proposed a bill to stop San Francisco's larger supermarkets from offering people nonbiodegradable plastic bags. It was put on hold for a year and a half, while grocery chains ran a voluntary program to cut back on their use of plastic.
*(Note: earlier versions of this story had quoted the numbers cited by Mr. Mirkarimi incorrectly. We apologize for the error.)
"I know it's been successful, I know the numbers will be very impressive," said Peter Larkin of the California Grocers Association, in January.
Larkin's group said the number of plastic bags used in San Francisco dropped by 7.6 million from 2005 to 2006.
But the association had pledged to meet a goal of 10 million bags. And the city's department of the environment said even the 7.6 million claim could not be verified.
"We had a handshake agreement. They could have impressed us and did the right thing for the environment," said San Francisco Mayor Gavin Newsom.
So, today, Mirkarimi is back, urging that stores that do more than $2 million a year in business be fined unless they stop offering people nonbiodegradable plastic bags. His bill has six of the city's 11 supervisors as co-sponsors.
"I think this is one measure emblematic of how a city government can step up to the plate," he said. "There's a lot we can do locally."