The animal kingdom is huge. It contains more than a million species. Most are small - much smaller than your eyes can see. But a few, like elephants and whales, are giants. All animals have one thing in common. They cannot make food. They need to eat plants or other animals to survive. All animals also need oxygen to survive. Animals include everything from ants, spiders, and worms to fish, lizards, bears, dogs, birds, cats, and humans.