We, the inhabitants of Earth, are just like people renting a house. Construction began on our home about 4.6 billion years ago--long before we got here. The floor is made of the Earth's crust, the roof is our planet's mountainous terrain, and the various rooms are the continents... In our home, the balance is fragile. If we use too much water, take bad care of it, or have an awful relationship with our neighbors, this beautiful building could collapse. Preserving it means we won't have to move to another planet some day!