Let them distill a salt water solution and taste the water before and after distillation. This not only makes them aware of the movement of water in our closed system but , it gives them an opportunity to understand why the oceans are salty. We know that water is called the universal solvent. This means that it can dissove more materials than any other know substance. Our concern, with this unit, is industrial wastes which includes heavy metals and other industrial contaminants that must be prevented from entering our water supply.
Earth is a closed system which means that almost nothing new can impact our system.
This makes it important for us to decide how to use this resource and others wisely as well as safely. The earth is like an egg. The shell of an egg can be compared to the thin outer crust of the earth. We are interested in knowing about the crust for various reasons, one being that we live on the crust and must learn how to use the resources to their maximum potential to support life forms. The mantle can be compared to the white of an egg.