Coloring is an activity that children worldwide have long enjoyed. With a picture already made for them, kids simply choose a crayon, pencil or marker of their favorite color and fill it in.

But why should children get to have all the fun? Now they don’t. More and more adults are enjoying relaxing hours of coloring.

This is thanks to a new book craze in the United States and in other countries around the world -- an adult coloring book craze. In December, five of the top ten best-selling books in the United States were coloring books made for adults.

Artist Trina Lucido creates coloring books. To find inspiration for her drawings, she says she doesn’t have to look far.

"One day I saw a beautiful leaf and -- boom! -- I did a whole page on leaves. So, some came from sketchbooks. Some were just one-word inspiration and some I just saw something on the ground and that was my inspiration that day."

Trina Lucido's latest coloring book contains 40 pages filled with hand-sketched drawings of flowers, birds and geometric shapes. It also has spaces to journal -- in other words, to write about what you’re doing or thinking. Journaling has long been an exercise in many types of therapy.

Lucido says the “big idea” behind her latest book is to help encourage people to do something creative.

"It’s called Color, Write and Create. I wanted to do something that would encourage people to be creative. When I do art journaling, people see my journals and (they go), ‘Oh my gosh! I wish I were creative.' And that makes me sad because I think everybody is creative. So the idea behind this book, the big idea is to encourage people to be creative..."

Lucido says the positive feedback from her customers is encouraging. She says that many of her customers are buying the coloring books for others who are sick or having challenges.

Lucido adds that the act of coloring becomes almost healing, or as she says, “therapeutic.”

“Something else I heard from people, they are buying these for people who are either ill or going through a rough time, and they feel that this is something that kind of helps them, almost therapeutically. "

Art therapy experts have said in several articles that coloring can reduce stress by focusing you on a simple task. When you are filling in a flower with a vibrant purple color, you focus on the flower and not on your troubles.

Experts add that coloring can bring out your imagination and can take you back to your childhood.

Psychologists add that coloring activates your brain in a healthy way. The shape and form of the drawings activate one side of the brain, while the color and texture of the drawing material activates the other.

Coloring also exercises a person’s fine motor skills. But people drawn to coloring books probably are not thinking of exercising their fine motor skills.

Susan Jackson is a big fan of Lucido’s coloring books. Jackson stands in a local store in the Washington, D.C. area. She holds one of Lucido’s coloring books in her hands. As she flips through the book, she says she is drawn to their beauty and authenticity. Sitting down and coloring, Jackson adds, relaxes her and just makes her feel happy.

"Oh, it is very relaxing. Any stress just sort of falls away. It frees your mind. And it's not as daunting as trying to fill a blank page. The pictures, I think, are just happy. They just make me feel happy. I love her little birds. There is just something about them. They make me smile. So I'm saving the bird pictures to the very end, I think."

For now, there is strong demand for adult coloring books. That fact keeps Lucido happy and very busy.