If a plastic is labelled "biodegradable", then the company that produces the bag is saying that if the plastic is left outside, it will decompose, which means that it will be broken down into simple chemicals that end up in soil, plants, rocks or gases in the air. The actual process of breaking down the plastic usually involves a combination of being in direct sunlight (sunlight weakens the chemical bonds that hold the plastic together), heat (heating up a plastic makes it easier to break down), and being eaten by bacteria.
To make sure that these bags break down and get eaten by bacteria, companies make them out of natural products - starch (potatoes), cellulose (paper), lactose (milk) or similar chemicals. If this kind of biodegradable bag is left out in the sun on a hot day, it will decompose over a few weeks or even days. For comparison, a non-biodegradable plastic bag will stay around for thousands for years.
Unfortunately, a lot of plastic bags that get thrown away do not end up in places where there is a lot of sunlight, heat, or even bacteria to eat them. The bags might sink to the bottom of the ocean, or be buried in the ground or a landfill. In these cases, the bags will stay around for years and simply pile up like any other trash. Recycling the bags could solve this problem, because recycling and composting centers can make sure that the bags are heated and placed in areas where they will degrade properly.
Making plastic that will be strong enough to hold groceries, but still degrade after being thrown away is a complicated science problem that many chemists are working on today. Some scientists are trying to make better plastics, while others try to create better bacteria to eat the plastic afterwards.
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