The jar idea is good, but first you should give the students some experiences with what they're going to see in the jar. Give them samples of sand, silt, and clay and have them make observations. Which one is the roughest? Which one has the largest pieces? Even before this, though, give them some sandstones to rub together so they can see that sand comes from larger rocks. They can look at the grains of sand under a magnifying glass to see that they are really just very tiny rocks.
You could also have them dig for soil in different areas of the school yard so they can see that soil isn't always the same. Have them compare soil under a tree, in a flower bed, and on the playground. Kids love digging in the dirt!