Tote in, Tote Out
First, let's start by giving credit where credit is due: Plastic bags (the ones they give you at checkout) are, in many ways, awesome. Lightweight and extremely scrunch-able, they're still strong enough to carry heavyweight cartons of milk, frozen fish and that candy bar you're buying to reward yourself for going through the painful process of shopping. On top of that, they're always there, at the counter, waiting to be stuffed with your items. No need to remember, on top of your shopping list, to bring the flotilla of grimy sacks you've been accumulating in your closet.
There are just so many ways to forget those bags of yours — first, after you put away groceries at home, you have to consolidate them and store them somewhere so they don't pollute your living space. Then, before the next trip to the store you have to unearth them again, and when you get to the checkout line you have to be fast enough to outdraw the cashiers. Over time, these pros have honed to a blinding speed their ability to open a new plastic bag and start filling it before you've had a chance to put down that rag with the headlines shouting about the latest alien abduction of the royal family.
Plastic bags are pernicious, and for reasons too complicated to enumerate, a lot of them seem to end up joining the estimated 269,000 tons (244,033 metric tons) of plastic currently swirling around in the ocean, getting accidentally ingested by sea life and generally wreaking havoc on the ecosystem [source: Schwartz]. So, you know, just bring your own bags — it's not that hard. Do it for the fish.