Emily, a reader in Silver Spring, wrote to ask, "What is an animal?" It may sound like a simple question, but it isn't really.

How would you answer? Would you say, "That's easy. An animal is something alive, like a dog or a cat or a horse."

You'd be right, of course. But the complete answer is more complicated. Animals are living things, or organisms. But plants are organisms too. Think about plants for a minute. They're alive and growing, but they don't move from place to place under their own power. Plants use sunlight and water to grow -- a process called photosynthesis -- but they don't eat food the way a cat or a dog or a horse does. So two things that set animals apart from plants are: 1) Most animals can move by themselves. 2) Animals don't produce food by photosynthesis.

Animals come in an incredible variety of sizes and shapes. A squishy sea sponge is an animal. So is a turtle. So is a seagull. So is an elephant. So is a centipede. And so is a human being.

Biologists have a system for organizing, or classifying, living organisms into different groups. This system has seven parts: kingdom, phylum, class, order, family, genus and species. (There's an old-fashioned memory trick that people use to remember these groups. It's the sentence, "King Philip Came Over From Geneva, Switzerland." Get it? The first letter of each word is a reminder of the first letter in the classification system.

Let's list how you fit into this classification system. You're a member of the animal kingdom. Your phylum is vertebrate. Your class is mammal. Your order is primate. Your family: hominidae. Your genus: homo. Your species: sapiens.

If some biologists read this information about you, they'd know several things: that you're a living organism, because you're a member of the animal kingdom. That that you have bones, because you're a vertebrate, and all vertebrates have bones. That you're warm-blooded and have hair, because you're a mammal. That you're related to apes and monkeys because you're a primate. And they'd know you are a human being because of your genus (homo, which means man) and your species (sapiens, which means wise).

If all of this seems complicated, think about this: On our planet, there are more than 10 million species of plants and animals. Without a classification system, it would impossible to keep track of them all.

Most creatures that pop into your mind when you hear the word "animal" are vertebrates. Fish, reptiles, amphibians, birds and mammals all have bones. Fish, reptiles and amphibians are cold-blooded animals. Scientists believe that fish were the first animals to develop bones. Some primitive fish, like sharks, have skeletons made out of a tough material called cartilage, but all the rest have bones. Fish take in oxygen from the water through gills.

Amphibians were the first animals to develop legs. Frogs, toads and salamanders are amphibians. When young, they live in the water and use gills to breathe. As adults, they develop legs and lungs, and live on land.

Reptiles are scaly animals, such as snakes, lizards, turtles and alligators. They're cold-blooded like fish and amphibians. Unlike amphibians, they have lungs and breathe oxygen from the air from the time they are hatched from their eggs. Reptiles have been around for a very, very long time -- about 300 million years! Dinosaurs were reptiles. But turtles were here before the dinosaurs and have outlasted them.

Birds are the only animals with feathers. They're warm-blooded. They have bills but no teeth. They hatch their young out of eggs. They have two legs and two wings, and most know how to fly. They have hollow bones that help keep their bodies light enough to take off into the air. Like you, birds have lungs and breathe oxygen. At this time of year, you might observe some species of birds migrating from their summer homes in the north to their winter homes in the south.

The animals we have most in common with are mammals. Like us, they're warm and fuzzy. That's because all 12,000 species of mammals -- including whales -- are warm-blooded and have hair in some form. Mammals all have backbones and lungs and bear live young. They have four appendages -- two arms and two legs, in the case of human beings.

Mammals have another important characteristic in common: When young, mammals are fed on milk produced by their mothers' bodies. Mammal babies -- especially humans -- depend on their parents for longer periods than other animals to teach them many of the skills needed to survive.

As you can see, animals are some of the most interesting and complex organisms on the planet. You should know; you're one of them!

Tips for Parents

One of the best places for studying animals is in the District: the Smithsonian's National Zoological Park. In addition to visiting the pandas and watching the seals play, you and your children can try interactive learning at the Discovery Room or Birdlab, open on weekends from 10 to 2. Herplab, the Zoo's learning reptile and amphibian learning lab, is closed now but will reopen Oct. 12. For zoo hours, parking fees and special programs, call the recorded information line: 202-673-4800.

Catherine O'Neill is a children's writer.