Because in the landfill, things do not really go away. Most waste in the USA is disposed of in a sanitary landfill. A landfill is basically a huge hole in the ground where our waste is buried. The hole is lined with some type of plastic liner. Buried waste changes over time.
Materials biodegrade or break down when they are exposed to soil, moisture, air, and sunlight.
Research indicates that biodegradation is greatly inhibited in landfills. For example, in one study, a newspaper that was dug up after 30 years was still readable. Divide students into groups of three or four. Line the bottom of the shoe box with foil to trap moisture. With a toothpick, label where each item was planted.