What is the deal with the imaginary friends that kids come up with?

Dear Matthew Alice:

When my four-year-old gets in trouble for doing something he shouldn't, he usually responds, "Wasn't me. Ziggy did it." We don't know anyone called Ziggy. He's been talking about this Ziggy since he's been able to talk. Before, it was "Don't sit there, you'll sit on Ziggy!" or stuff like that. My nine-year-old daughter used to have an imaginary friend whom she called Invisible Cody that she played with and talked to when she was alone. Just what's the deal with these imaginary friends kids come up with? Should I be worried? Should I call Ghostbusters? Should I call a kiddyshrink?

-- Lori, the net

Calm down, Mom. Your kids are dead-on average, according to most research by child psychologists. Percentage of kids who have imaginary friends: around 50. Typical age of onset: three or four. Typical age when they go away (or at least aren't talked about anymore): nine or ten. Do kids know they're not flesh-and-blood real? Of course, even when they insist that their friends get a chair and plate at the dining table or that you hold their invisible hands when you cross the street.

Kids invent them for companionship (Cody), to help deal with new life situations or kid-type fears (Ziggy), or maybe just for fun. Best of all, an imaginary friend can be given any qualities your kid desires (superhero, helpless little sister, playground pal). It's all part of learning to socialize and cope and only becomes a problem when a child's fantasy life takes over his real life or when this "friend" seems more harmful than quirky and benign. A kid's imagination is a thing of wonder. I'd rather hang out with Ziggy and Cody than sit on my can fantasizing with grown-up imaginary friends in an Internet chat room.