I discovered that my 4 year-old daughter, Rosa, had an imaginary friend the day I asked why my sofa was covered in lipstick smears. "Dotty did it," she explained. When I asked who Dotty was, Rosa informed me that she was her sister. This was news to me. Over the next few months, I got to know Dotty pretty well: her likes, dislikes, weak-nesses, and inclinations. And when she wasn't breaking into my makeup stash, I was happy to have her around; she was free, endless (and quiet!) entertainment for Rosa.

I'm far from the only mom to be hosting invisible houseguests: 65 percent of kids under 7 have had imaginary friends at some point, according to a study by Marjorie Taylor, Ph.D., author ofImaginary Companions and the Children Who Create Them. And these friends will most likely pop up during the preschool years, when kids have developed the ability for more complex forms of pretend play. Three- and 4-year-olds are also beginning to notice the reasoning behind other people's actions (my brother threw the juice box because he didn't like the taste), which is a critical skill for understanding the perspective of another person -- real or imaginary.

There's no one-size-fits-all standard for these dreamed-up allies. They can be human or completely fantastical. Some stick around for years; others get traded in every few months for a newer model. But what all imaginary friends have in common, says Dr. Taylor, are the benefits they offer to children, helping with everything from problem-solving skills to emotional well-being. Read on for answers to your questions about these imaginary friends.

Will my child still play with his real friends?

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