Check out a book about the earth. Here's one of my favorites:
On Earth by G. Brian Karas
And here are some others:
Earth Our Planet in Space by Seymour Simon
The Down-to-Earth Guide to Global Warming by Laurie David and Cambria Gordon
Global Warming : the Threat of Earth's Changing Climate by Laurence Pringle.
You're Aboard Spaceship Earth by Patricia Lauber
True Green Kids : 100 Things You Can Do to Save the Planet by Kim McKay and Jenny Bonnin
Why are the Ice Caps Melting? : the Dangers of Global Warming by Anne Rockwell
Happy Earth Day!