Decomposers essentially break down the stuff that dies of the stuff produced by living things. So if nothing ever got broken down you would have to walk to school through thick layers of dead stuff and poop. Think of walking through dead dinos and dino poop - ugh! Also, when this stuff is broken down into its parts it lets the materials be used again by new plants and animals and other living things. (A decomposer is an organism, often a bacterium or fungus, that feeds on and breaks down dead plant or animal matter, thus making organic nutrients available to the ecosystem.)