Coloring books are where most kids start to develop their artistic dexterity. They are available relatively cheap, they are often related to cartoon characters or objects familiar to very young children. They provide guidelines for children who are learning to control their hand movements by "coloring inside the lines".
Developing New Skills. It is very important for children to develop their fine motor skills. Most children do not even realize that by coloring they are accomplishing this task. Having children complete coloring projects and printable coloring sheets will help children with many skills that will be valuable later in life.
According to Wikipedia: A coloring book (or colouring book) is a type of book containing line art for a reader to add color using crayons, colored pencils, marker pens, paint or other artistic media. Coloring books are generally used by children, though coloring books for adults are also available. They cover a wide range of subjects, from simple stories for children to intricate or abstract designs for adults. The golden age of coloring books is considered to be the 1960s.
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The first German airline to use heavier than air aircraft was Deutsche Luft-Reederei established in 1917 which started operating in February 1919. In its first year, the D. L. R. operated regularly scheduled flights on routes with a combined length of nearly 1000 miles. By 1921 the D. L. R. network was more than 3000 km (1865 miles) long, and included destinations in the Netherlands, Scandinavia and the Baltic Republics. Another important German airline was Junkers Luftverkehr, which began operations in 1921. It was a division of the aircraft manufacturer Junkers, which became a separate company in 1924. It operated joint-venture airlines in Austria, Denmark, Estonia, Finland, Hungary, Latvia, Norway, Poland, Sweden and Switzerland. 
People are making the adult coloring books bestsellers on Amazon! At the time of this writing, eight of the top twenty books on the bestsellers list, are coloring books for adults. There must be something behind this recent growth in interest.
Nowadays the World Wide Web made simple getting necessary info. And coloring pages reap benefits of the Internet enhancement.
Today the Internet simplified looking for info. And coloring pages took advantage of the Internet progress.
As she is leaving, her final request is often for me to print a few more pages for her to take home or even a large batch so she can share with her friends when they visit. Of course, as always I am too happy to oblige. Especially as it's so cheap to print them out. Few things are so cheap but can still make a child so happy these days.