single-use plastic bags aren’t the worst environmental choice at the supermarket—they’re the best. High-density polyethylene bags are a marvel of economic, engineering and environmental efficiency. They’re cheap, convenient, waterproof, strong enough to hold groceries but thin and light enough to make and transport using scant energy, water or other resources. Though they’re called single-use, most people reuse them, typically as trash-can liners. When governments ban them, consumers buy thicker substitutes with a bigger carbon footprint.