Ask them if they understand what happens to all of their garbage. Explain to them that there are still countries where wastewater still flows in gutters on the side of the city streets. Tell them how trash is fed to pigs that are then slaughtered and eaten. Many students have never thought about how the ancient communities took care of their waste. This is a great factor in why population growth has escalated.
Population growth is a great unit to cross into math with.
There are so many things that can be done to the numbers that demonstrate basic math principals. I have included a few story problems that I wrote. These are just examples, some fourth grade level students may not be able to do these yet, but others may have no problems. An island population in the middle of the Pacific Ocean has just been discovered. The people of this island are being studied.