What is a Fossil?

My group decided to make 6 different varieties of the sand/plaster mix to see which would be best. We started with 25 ml of sand and 100 ml of plaster and ended with 150 ml of sand and 100 ml of plaster and I found that we still could have added even more sand. All of the samples were extremely hard to break open. If we didn't have a hammer or pick ax I don't think they would have opened. The final sample that had 150 ml of sand was hard to chip away but easy to chisel and scrape at. Since I probably wouldn't want to give young kids a hammer, that recipe would work best but with more sand. As we add more sand to the constant plaster amount, I found that it was easier to get to the fossil.