We made a line across the room and pegged various times on it to make a timeline. Then we were given items and had to put them where we thought they matched with the time to break down. We even added our own item: a human being! haha. (he's not slacking off - he's part of our display).
Everything breaks down but some things do so much faster than others and some take a reallllly long time - hundreds to millions of years. The point is: don't buy things that are on the longer end of the timeline as if, for whatever reason, they end up in the environment, they'll be there longer than they should.