When Rosie came downstairs to have breakfast the other day, she saw something little crawling across the dining room floor. Her cat saw it, too, and was very interested.

"I wonder what that is," thought Rosie. "I didn't think there were many bugs around in the wintertime."

Rosie picked up her cat and took a closer look at the tiny creature scuttling across the floor. It was a spider. But Rosie didn't squash it. "I'd like to see it make a web," she thought.

You may have thought a spider was an insect. But it's not. Insects have six legs; spiders have eight. Spiders belong to a group called arachnida, which also includes scorpions, daddy longlegs, mites and ticks.

The arachnida get their name from a Greek myth about a woman named Arachne. Arachne was a seamstress who sewed the most beautiful tapestries in the world. This talent made one of the Greek goddesses jealous, so she turned poor Arachne into a spider. But Arachne kept on spinning lovely creations even after she became an eight-legged creature.

Spiders are common. There are about 30,000 different kinds, or species, on earth. They have adapted, or gotten used to, many different environments. They live in woodpiles and prairie grasses, deserts and mountains, caves and fields. Some species are particularly fond of hanging around in the dark corners of basements and closets in people's houses. Others prefer gardens and spin beautiful webs among the plants.

Spiders feed on insects. They have venom, or poison, to help them capture and eat their prey. When spiders bite, the poison paralyzes the prey. But spiders rarely bite people, and only a few kinds have really dangerous venom. As far as humans are concerned, most spiders are completely harmless.

Not all spiders build webs. Some go hunting; others lie in wait for their prey. But when you hear the word "spider," you probably think of the word "web," too.

Spiders do create amazing webs. They build them with silk that they spin out of their own bodies. Different kinds of spiders build different kinds of webs. Many are the familiar, more-or-less circular kind. These are called "orb webs." Other webs are shaped like funnels or like bowls. Some look like small sheets spread out to dry on the grass.

A spider spins silk from its abdomen, the round, lower part of its body. The silk leaves the spider's body through little tubes called spinnerets. It can make different kinds of silk for different uses. Need a length of dry silk to make the support beams of a web? Here's some. Need some sticky silk to build a trap? Coming right up. Both kinds of silk leave the spider's body as a liquid; the fluid hardens when it comes in contact with the air.

Spiders don't have to learn how to build webs. They do it by instinct, and all members of a particular spider species build the same kind of webs. You've probably seen examples of the circular orb webs. Those complicated structures begin with just one strand called a bridge thread. Let's say a spider is making a web on an old chair. It sits on one leg of the chair, ready to start building. To make the bridge thread, it releases a strand of silk from a spinneret and launches itself into the air.

The spider floats through the air until it lands on one of the rungs of the chair. The silk is now stretched between two parts of the chair. The spider scurries back across this strand, doubling it up with another piece of silk to make it strong. The bridge thread is finished.

Now the spider begins to build interconnected triangles that will be the web's underpinnings. They're sort of like the spokes of a bicycle wheel. Once the spokes are done, the spider makes the spiral part of its web. It does a test run first, starting at the middle and walking around and around to make a spiral of dry silk. This temporary spiral gives the spider a support structure to walk on when it makes its permanent, sticky web.

Once the dry spiral is complete, the spider starts all over again. It retraces its steps, eating the dry silk and replacing it with sticky silk for catching prey. Finally, it sits in the web and waits for insects to come along. Dinnertime!

There are people who are scared to death of spiders. There's a fancy scientific name for this fear: arachnaphobia. You probably remember that there was a scary movie by that name playing in theaters last year.

But Rosie doesn't have arachnaphobia. She's interested, not scared, when she sees something like a spider, or even a snake. After she sees the spider in her dining room, she runs upstairs to get her magnifying glass.

"I want to take a closer look," she says. "Maybe I'll even see it spinning its web." Tips for Parents

Things that seem icky to adults can fascinate children. You can help encourage your kids' interest in science and the natural world by providing them with some basic tools and resources: some kids' science books and magazines, a good-quality magnifying glass, a simple microscope. Need suggestions for activities? Check out "Scienceworks," (Addison-Wesley) a book of safe and fascinating experiments to do at home, designed by the staff at the interactive Ontario Science Centre in Toronto.

Catherine O'Neill is a freelance children's writer.